diff --git a/README.md b/README.md
index 6c42498..f56e437 100644
--- a/README.md
+++ b/README.md
@@ -29,14 +29,14 @@ $ python -m settngs
 Hello lordwelch
 $ python -m settngs -v
 Hello lordwelch
-merged_namespace.values.example_verbose=True
+merged_namespace.values.Example_Group__verbose=True
 $ python -m settngs -v -s
 Hello lordwelch
 Successfully saved settings to settings.json
-merged_namespace.values.example_verbose=True
+merged_namespace.values.Example_Group__verbose=True
 $ python -m settngs
 Hello lordwelch
-merged_namespace.values.example_verbose=True
+merged_namespace.values.Example_Group__verbose=True
 $ cat >settings.json << EOF
 {
   "example": {
diff --git a/settngs/__init__.py b/settngs/__init__.py
index 569eac9..f5a11cb 100644
--- a/settngs/__init__.py
+++ b/settngs/__init__.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import argparse
+import copy
 import json
 import logging
 import pathlib
@@ -247,7 +248,7 @@ class Setting:
         if not dest_name.isidentifier():
             raise Exception(f'Cannot use {dest_name} in a namespace')
 
-        internal_name = f'{prefix}_{dest_name}'.lstrip('_')
+        internal_name = f'{prefix}__{dest_name}'.lstrip('_')
         return internal_name, dest_name, flag
 
     def filter_argparse_kwargs(self) -> dict[str, Any]:
@@ -376,11 +377,34 @@ def get_options(config: Config[T], group: str) -> dict[str, Any]:
                 if name in internal_names:
                     values[internal_names[name].dest] = value
                 else:
-                    values[removeprefix(name, f'{group}_')] = value
-
+                    values[removeprefix(name, f'{group}').lstrip('_')] = value
     return values
 
 
+def get_groups(values: Values | Namespace | TypedNS) -> list[str]:
+    if isinstance(values, dict):
+        return [x[0] for x in values.items() if isinstance(x[1], dict)]
+    if isinstance(values, Namespace):
+        groups = set()
+        for name in values.__dict__:
+            if '__' in name:
+                group, _, _ = name.partition('__')
+                groups.add(group.replace('_', ' '))
+            else:
+                groups.add('')
+        return list(groups)
+    return []
+
+
+def _get_internal_definitions(config: Config[T], persistent: bool) -> Definitions:
+    definitions = copy.deepcopy(dict(config.definitions))
+    if persistent:
+        for group_name in get_groups(config.values):
+            if group_name not in definitions:
+                definitions[group_name] = Group(True, {})
+    return defaultdict(lambda: Group(False, {}), definitions)
+
+
 def normalize_config(
     config: Config[T],
     file: bool = False,
@@ -398,18 +422,19 @@ def normalize_config(
         file: Include file options
         cmdline: Include cmdline options
         default: Include default values in the returned Config object
-        persistent: Include unknown keys in persistent groups
+        persistent: Include unknown keys in persistent groups and unknown groups
     """
 
     if not file and not cmdline:
         raise ValueError('Invalid parameters: you must set either file or cmdline to True')
 
     normalized: Values = {}
-    options, definitions = config
+    options = config.values
+    definitions = _get_internal_definitions(config=config, persistent=persistent)
     for group_name, group in definitions.items():
         group_options = {}
         if group.persistent and persistent:
-            group_options = get_options(config, group_name)
+            group_options = get_options(Config(options, definitions), group_name)
         for setting_name, setting in group.v.items():
             if (setting.cmdline and cmdline) or (setting.file and file):
                 # Ensures the option exists with the default if not already set
@@ -424,7 +449,8 @@ def normalize_config(
                 # Setting type (file or cmdline) has not been requested and should be removed for persistent groups
                 del group_options[setting_name]
         normalized[group_name] = group_options
-    return Config(normalized, definitions)
+
+    return Config(normalized, config.definitions)
 
 
 def parse_file(definitions: Definitions, filename: pathlib.Path) -> tuple[Config[Values], bool]:
@@ -467,7 +493,7 @@ def clean_config(
         file: Include file options
         cmdline: Include cmdline options
         default: Include default values in the returned Config object
-        persistent: Include unknown keys in persistent groups
+        persistent: Include unknown keys in persistent groups and unknown groups
     """
 
     cleaned, _ = normalize_config(config, file=file, cmdline=cmdline, default=default, persistent=persistent)
@@ -493,24 +519,22 @@ def get_namespace(
         file: Include file options
         cmdline: Include cmdline options
         default: Include default values in the returned Config object
-        persistent: Include unknown keys in persistent groups
+        persistent: Include unknown keys in persistent groups and unknown groups
     """
-
     if not file and not cmdline:
         raise ValueError('Invalid parameters: you must set either file or cmdline to True')
 
     options: Values
-    definitions: Definitions
+    definitions = _get_internal_definitions(config=config, persistent=persistent)
     if isinstance(config.values, dict):
         options = config.values
-        definitions = config.definitions
     else:
         cfg = normalize_config(config, file=file, cmdline=cmdline, default=default, persistent=persistent)
-        options, definitions = cfg
+        options = cfg.values
     namespace = Namespace()
     for group_name, group in definitions.items():
 
-        group_options = get_options(config, group_name)
+        group_options = get_options(Config(options, definitions), group_name)
         if group.persistent and persistent:
             for name, value in group_options.items():
                 if name in group.v:
@@ -519,7 +543,7 @@ def get_namespace(
                     internal_name = group.v[name].internal_name
                 else:
                     setting_file = setting_cmdline = True
-                    internal_name, is_default = f'{group_name}_' + sanitize_name(name), None
+                    internal_name, is_default = f'{group_name}__' + sanitize_name(name), None
 
                 if ((setting_cmdline and cmdline) or (setting_file and file)) and (not is_default or default):
                     setattr(namespace, internal_name, value)
@@ -531,7 +555,7 @@ def get_namespace(
                 if not is_default or default:
                     # User has set a custom value or has requested the default value
                     setattr(namespace, setting.internal_name, value)
-    return Config(namespace, definitions)
+    return Config(namespace, config.definitions)
 
 
 def save_file(
@@ -660,10 +684,11 @@ class Manager:
         self.description = description
         self.epilog = epilog
 
+        self.definitions: Definitions
         if isinstance(definitions, Config):
-            self.definitions = definitions.definitions
+            self.definitions = defaultdict(lambda: Group(False, {}), dict(definitions.definitions) or {})
         else:
-            self.definitions = defaultdict(lambda: Group(False, {}), definitions or {})
+            self.definitions = defaultdict(lambda: Group(False, {}), dict(definitions or {}))
 
         self.exclusive_group = False
         self.current_group_name = ''
@@ -766,7 +791,7 @@ class Manager:
             file: Include file options
             cmdline: Include cmdline options
             default: Include default values in the returned Config object
-            persistent: Include unknown keys in persistent groups
+            persistent: Include unknown keys in persistent groups and unknown groups
         """
 
         return normalize_config(
@@ -779,7 +804,7 @@ class Manager:
 
     def get_namespace(
         self,
-        config: Values | Config[Values],
+        config: T | Config[T],
         file: bool = False,
         cmdline: bool = False,
         default: bool = True,
@@ -795,7 +820,7 @@ class Manager:
             file: Include file options
             cmdline: Include cmdline options
             default: Include default values in the returned Config object
-            persistent: Include unknown keys in persistent groups
+            persistent: Include unknown keys in persistent groups and unknown groups
         """
 
         return get_namespace(
@@ -915,13 +940,13 @@ def _main(args: list[str] | None = None) -> None:
     merged_namespace = manager.get_namespace(merged_config, file=True, cmdline=True)
 
     print(f'Hello {merged_config.values["Example Group"]["hello"]}')  # noqa: T201
-    if merged_namespace.values.Example_Group_save:
+    if merged_namespace.values.Example_Group__save:
         if manager.save_file(merged_config, settings_path):
             print(f'Successfully saved settings to {settings_path}')  # noqa: T201
         else:  # pragma: no cover
             print(f'Failed saving settings to a {settings_path}')  # noqa: T201
-    if merged_namespace.values.Example_Group_verbose:
-        print(f'{merged_namespace.values.Example_Group_verbose=}')  # noqa: T201
+    if merged_namespace.values.Example_Group__verbose:
+        print(f'{merged_namespace.values.Example_Group__verbose=}')  # noqa: T201
 
 
 if __name__ == '__main__':
diff --git a/setup.cfg b/setup.cfg
index cf49039..4dc8e63 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -90,6 +90,7 @@ fail_under = 95
 [mypy]
 check_untyped_defs = true
 disallow_any_generics = true
+warn_return_any = true
 disallow_incomplete_defs = true
 disallow_untyped_defs = true
 no_implicit_optional = true
@@ -97,7 +98,9 @@ warn_redundant_casts = true
 warn_unused_ignores = true
 
 [mypy-testing.*]
+warn_return_any = false
 disallow_untyped_defs = false
 
 [mypy-tests.*]
+warn_return_any = false
 disallow_untyped_defs = false
diff --git a/testing/settngs.py b/testing/settngs.py
index 7a8a3e9..f113742 100644
--- a/testing/settngs.py
+++ b/testing/settngs.py
@@ -24,22 +24,22 @@ example: list[tuple[list[str], str, str]] = [
     ),
     (
         ['-v'],
-        'Hello lordwelch\nmerged_namespace.values.Example_Group_verbose=True\n',
+        'Hello lordwelch\nmerged_namespace.values.Example_Group__verbose=True\n',
         '{\n  "Example Group": {\n    "hello": "lordwelch",\n    "verbose": false\n  },\n  "persistent": {\n    "test": false\n  }\n}\n',
     ),
     (
         ['-v', '-s'],
-        'Hello lordwelch\nSuccessfully saved settings to settings.json\nmerged_namespace.values.Example_Group_verbose=True\n',
+        'Hello lordwelch\nSuccessfully saved settings to settings.json\nmerged_namespace.values.Example_Group__verbose=True\n',
         '{\n  "Example Group": {\n    "hello": "lordwelch",\n    "verbose": true\n  },\n  "persistent": {\n    "test": false\n  }\n}\n',
     ),
     (
         [],
-        'Hello lordwelch\nmerged_namespace.values.Example_Group_verbose=True\n',
+        'Hello lordwelch\nmerged_namespace.values.Example_Group__verbose=True\n',
         '{\n  "Example Group": {\n    "hello": "lordwelch",\n    "verbose": true\n  },\n  "persistent": {\n    "test": false\n  }\n}\n',
     ),
     (
         ['manual settings.json'],
-        'Hello lordwelch\nmerged_namespace.values.Example_Group_verbose=True\n',
+        'Hello lordwelch\nmerged_namespace.values.Example_Group__verbose=True\n',
         '{\n  "Example Group": {\n    "hello": "lordwelch",\n    "verbose": true\n  },\n  "persistent": {\n    "test": false,\n    "hello": "world"\n  }\n}\n',
     ),
     (
@@ -84,7 +84,7 @@ success = [
             'flag': True,
             'group': 'tst',
             'help': None,
-            'internal_name': 'tst_test_setting',  # Should almost always be "{group}_{dest}"
+            'internal_name': 'tst__test_setting',  # Should almost always be "{group}_{dest}"
             'metavar': 'TEST_SETTING',  # Set manually so argparse doesn't use TST_TEST
             'nargs': None,
             'required': None,
@@ -95,7 +95,7 @@ success = [
                 'choices': None,
                 'const': None,
                 'default': None,
-                'dest': 'tst_test_setting',
+                'dest': 'tst__test_setting',
                 'help': None,
                 'metavar': 'TEST_SETTING',
                 'nargs': None,
@@ -125,7 +125,7 @@ success = [
             'flag': True,
             'group': 'tst',
             'help': None,
-            'internal_name': 'tst_testing',  # Should almost always be "{group}_{dest}"
+            'internal_name': 'tst__testing',  # Should almost always be "{group}_{dest}"
             'metavar': 'TESTING',  # Set manually so argparse doesn't use TST_TEST
             'nargs': None,
             'required': None,
@@ -136,7 +136,7 @@ success = [
                 'choices': None,
                 'const': None,
                 'default': None,
-                'dest': 'tst_testing',
+                'dest': 'tst__testing',
                 'help': None,
                 'metavar': 'TESTING',
                 'nargs': None,
@@ -165,7 +165,7 @@ success = [
             'flag': True,
             'group': 'tst',
             'help': None,
-            'internal_name': 'tst_test',  # Should almost always be "{group}_{dest}"
+            'internal_name': 'tst__test',  # Should almost always be "{group}_{dest}"
             'metavar': 'TEST',  # Set manually so argparse doesn't use TST_TEST
             'nargs': None,
             'required': None,
@@ -176,7 +176,7 @@ success = [
                 'choices': None,
                 'const': None,
                 'default': None,
-                'dest': 'tst_test',
+                'dest': 'tst__test',
                 'help': None,
                 'metavar': 'TEST',
                 'nargs': None,
@@ -206,7 +206,7 @@ success = [
             'flag': True,
             'group': 'tst',
             'help': None,
-            'internal_name': 'tst_test',  # Should almost always be "{group}_{dest}"
+            'internal_name': 'tst__test',  # Should almost always be "{group}_{dest}"
             'metavar': None,  # store_true does not get a metavar
             'nargs': None,
             'required': None,
@@ -217,7 +217,7 @@ success = [
                 'choices': None,
                 'const': None,
                 'default': None,
-                'dest': 'tst_test',
+                'dest': 'tst__test',
                 'help': None,
                 'metavar': None,
                 'nargs': None,
@@ -246,7 +246,7 @@ success = [
             'flag': True,
             'group': 'tst',
             'help': None,
-            'internal_name': 'tst_test',
+            'internal_name': 'tst__test',
             'metavar': 'TEST',
             'nargs': None,
             'required': None,
@@ -257,7 +257,7 @@ success = [
                 'choices': None,
                 'const': None,
                 'default': None,
-                'dest': 'tst_test',
+                'dest': 'tst__test',
                 'help': None,
                 'metavar': 'TEST',
                 'nargs': None,
@@ -286,12 +286,12 @@ success = [
             'flag': False,
             'group': 'tst',
             'help': None,
-            'internal_name': 'tst_test',
+            'internal_name': 'tst__test',
             'metavar': 'TEST',
             'nargs': None,
             'required': None,
             'type': None,
-            'argparse_args': ('tst_test',),
+            'argparse_args': ('tst__test',),
             'argparse_kwargs': {
                 'action': None,
                 'choices': None,
diff --git a/tests/settngs_test.py b/tests/settngs_test.py
index 32bfd7b..5365e63 100644
--- a/tests/settngs_test.py
+++ b/tests/settngs_test.py
@@ -89,7 +89,7 @@ def test_exclusive_group(settngs_manager):
     settngs_manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'), exclusive_group=True)
     settngs_manager.create_argparser()
     args = settngs_manager.argparser.parse_args(['--test', 'never'])
-    assert args.tst_test == 'never'
+    assert args.tst__test == 'never'
 
     with pytest.raises(SystemExit):
         settngs_manager.add_group('tst', lambda parser: parser.add_setting('--test2', default='hello'), exclusive_group=True)
@@ -213,6 +213,29 @@ class TestValues:
         assert normalized['persistent']['hello'] == 'success'
         assert normalized['persistent']['world'] == 'world'
 
+    def test_unknown_group(self):
+        manager = settngs.Manager()
+        manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
+        manager.add_persistent_group('persistent', lambda parser: parser.add_setting('--world', default='world'))
+
+        manager_unknown = settngs.Manager()
+        manager_unknown.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
+        # This manager doesn't know about this group
+        # manager_unknown.add_persistent_group('persistent', lambda parser: parser.add_setting('--world', default='world'))
+
+        defaults = manager.defaults()
+        defaults.values['test'] = 'fail'  # type: ignore[assignment] # Not defined in manager, should be removed
+        defaults.values['persistent']['hello'] = 'success'  # Group is not defined in manager_unknown, should stay
+
+        normalized, _ = manager_unknown.normalize_config(defaults.values, file=True)
+
+        assert 'test' not in normalized
+        assert 'tst' in normalized
+        assert 'test' in normalized['tst']
+        assert normalized['tst']['test'] == 'hello'
+        assert normalized['persistent']['hello'] == 'success'
+        assert normalized['persistent']['world'] == 'world'
+
 
 class TestNamespace:
 
@@ -230,12 +253,12 @@ class TestNamespace:
     def test_get_defaults_group(self, settngs_manager):
         settngs_manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
         defaults, _ = settngs_manager.get_namespace(settngs_manager.defaults(), file=True, cmdline=True)
-        assert defaults.tst_test == 'hello'
+        assert defaults.tst__test == 'hello'
 
     def test_get_defaults_group_space(self, settngs_manager):
         settngs_manager.add_group('Testing tst', lambda parser: parser.add_setting('--test', default='hello'))
         defaults, _ = settngs_manager.get_namespace(settngs_manager.defaults(), file=True, cmdline=True)
-        assert defaults.Testing_tst_test == 'hello'
+        assert defaults.Testing_tst__test == 'hello'
 
     def test_cmdline_only(self, settngs_manager):
         settngs_manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello', file=False))
@@ -244,11 +267,11 @@ class TestNamespace:
         file_normalized, _ = settngs_manager.get_namespace(settngs_manager.normalize_config(settngs_manager.defaults(), file=True), file=True)
         cmdline_normalized, _ = settngs_manager.get_namespace(settngs_manager.normalize_config(settngs_manager.defaults(), cmdline=True), cmdline=True)
 
-        assert 'tst_test' not in file_normalized.__dict__
-        assert 'tst2_test2' in file_normalized.__dict__
+        assert 'tst__test' not in file_normalized.__dict__
+        assert 'tst2__test2' in file_normalized.__dict__
 
-        assert 'tst_test' in cmdline_normalized.__dict__
-        assert 'tst2_test2' not in cmdline_normalized.__dict__
+        assert 'tst__test' in cmdline_normalized.__dict__
+        assert 'tst2__test2' not in cmdline_normalized.__dict__
 
     def test_cmdline_only_persistent_group(self, settngs_manager):
         settngs_manager.add_persistent_group('tst', lambda parser: parser.add_setting('--test', default='hello', file=False))
@@ -257,11 +280,11 @@ class TestNamespace:
         file_normalized, _ = settngs_manager.get_namespace(settngs_manager.normalize_config(settngs_manager.defaults(), file=True), file=True)
         cmdline_normalized, _ = settngs_manager.get_namespace(settngs_manager.normalize_config(settngs_manager.defaults(), cmdline=True), cmdline=True)
 
-        assert 'tst_test' not in file_normalized.__dict__
-        assert 'tst2_test2' in file_normalized.__dict__
+        assert 'tst__test' not in file_normalized.__dict__
+        assert 'tst2__test2' in file_normalized.__dict__
 
-        assert 'tst_test' in cmdline_normalized.__dict__
-        assert 'tst2_test2' not in cmdline_normalized.__dict__
+        assert 'tst__test' in cmdline_normalized.__dict__
+        assert 'tst2__test2' not in cmdline_normalized.__dict__
 
     def test_normalize_defaults(self, settngs_manager):
         settngs_manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
@@ -273,12 +296,12 @@ class TestNamespace:
         assert defaults_normalized.values.__dict__ == {}
 
         non_defaults = settngs_manager.get_namespace(settngs_manager.defaults(), file=True, cmdline=True)
-        non_defaults.values.tst_test = 'world'
-        non_defaults.values.tst_persistent_test = 'world'
+        non_defaults.values.tst__test = 'world'
+        non_defaults.values.tst_persistent__test = 'world'
         non_defaults_normalized = settngs_manager.get_namespace(settngs_manager.normalize_config(non_defaults, file=True, default=False), file=True, default=False)
 
-        assert non_defaults_normalized.values.tst_test == 'world'
-        assert non_defaults_normalized.values.tst_persistent_test == 'world'
+        assert non_defaults_normalized.values.tst__test == 'world'
+        assert non_defaults_normalized.values.tst_persistent__test == 'world'
 
     def test_normalize(self, settngs_manager):
         settngs_manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
@@ -286,15 +309,37 @@ class TestNamespace:
 
         defaults = settngs_manager.get_namespace(settngs_manager.defaults(), file=True, cmdline=True)
         defaults.values.test = 'fail'  # Not defined in settngs_manager, should be removed
-        defaults.values.persistent_hello = 'success'  # Not defined in settngs_manager, should stay
+        defaults.values.persistent__hello = 'success'  # Not defined in settngs_manager, should stay
 
         normalized, _ = settngs_manager.get_namespace(settngs_manager.normalize_config(defaults, file=True), file=True)
 
         assert not hasattr(normalized, 'test')
-        assert hasattr(normalized, 'tst_test')
-        assert normalized.tst_test == 'hello'
-        assert normalized.persistent_hello == 'success'
-        assert normalized.persistent_world == 'world'
+        assert hasattr(normalized, 'tst__test')
+        assert normalized.tst__test == 'hello'
+        assert normalized.persistent__hello == 'success'
+        assert normalized.persistent__world == 'world'
+
+    def test_normalize_unknown(self, settngs_manager):
+        manager = settngs.Manager()
+        manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
+        manager.add_persistent_group('persistent', lambda parser: parser.add_setting('--world', default='world'))
+
+        manager_unknown = settngs.Manager()
+        manager_unknown.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
+        # This manager doesn't know about this group
+        # manager_unknown.add_persistent_group('persistent', lambda parser: parser.add_setting('--world', default='world'))
+
+        defaults = manager.get_namespace(manager.defaults(), file=True, cmdline=True)
+        defaults.values.test = 'fail'  # Not defined in manager, should be removed
+        defaults.values.persistent__hello = 'success'  # Not defined in manager, should stay
+
+        normalized, _ = manager_unknown.get_namespace(defaults.values, file=True)
+
+        assert not hasattr(normalized, 'test')
+        assert hasattr(normalized, 'tst__test')
+        assert normalized.tst__test == 'hello'
+        assert normalized.persistent__hello == 'success'
+        assert normalized.persistent__world == 'world'
 
 
 def test_get_namespace_with_namespace(settngs_manager):
@@ -306,7 +351,7 @@ def test_get_namespace_with_namespace(settngs_manager):
 def test_get_namespace_group(settngs_manager):
     settngs_manager.add_group('tst', lambda parser: parser.add_setting('--test', default='hello'))
     defaults, _ = settngs_manager.get_namespace(settngs_manager.defaults(), file=True)
-    assert defaults.tst_test == 'hello'
+    assert defaults.tst__test == 'hello'
 
 
 def test_clean_config(settngs_manager):
@@ -337,8 +382,8 @@ def test_parse_cmdline(settngs_manager):
 
 namespaces = (
     lambda definitions: settngs.Config({'tst': {'test': 'fail', 'test2': 'success'}}, definitions),
-    lambda definitions: settngs.Config(argparse.Namespace(tst_test='fail', tst_test2='success'), definitions),
-    lambda definitions: argparse.Namespace(tst_test='fail', tst_test2='success'),
+    lambda definitions: settngs.Config(argparse.Namespace(tst__test='fail', tst__test2='success'), definitions),
+    lambda definitions: argparse.Namespace(tst__test='fail', tst__test2='success'),
 )