diff --git a/schema.graphql b/schema.graphql
index 98f7454..443d4ed 100644
--- a/schema.graphql
+++ b/schema.graphql
@@ -3,6 +3,12 @@ schema {
   mutation: SchemaMutation
 }
 
+union AddTagResponse = AddTagSuccess | InvalidInputError | EnvironmentNotFoundError
+
+type AddTagSuccess implements Success {
+  message: String!
+}
+
 type BuilderError implements Error {
   message: String!
 }
@@ -31,6 +37,7 @@ type Environment {
   type: Type!
   packages: [Package!]!
   state: State
+  tags: [String!]!
   requested: DateTime
   buildStart: DateTime
   buildDone: DateTime
@@ -48,6 +55,7 @@ input EnvironmentInput {
   path: String!
   description: String!
   packages: [PackageInput!]!
+  tags: [String!] = null
 }
 
 type EnvironmentNotFoundError implements Error {
@@ -86,6 +94,7 @@ type PackageMultiVersion {
 type SchemaMutation {
   createEnvironment(env: EnvironmentInput!): CreateResponse!
   deleteEnvironment(name: String!, path: String!): DeleteResponse!
+  addTag(name: String!, path: String!, tag: String!): AddTagResponse!
   createFromModule(file: Upload!, modulePath: String!, environmentPath: String!): CreateResponse!
   updateFromModule(file: Upload!, modulePath: String!, environmentPath: String!): UpdateResponse!
 }
diff --git a/softpack_core/artifacts.py b/softpack_core/artifacts.py
index 987f3bb..30dc558 100644
--- a/softpack_core/artifacts.py
+++ b/softpack_core/artifacts.py
@@ -72,6 +72,7 @@ class Artifacts:
     builder_out = "builder.out"
     module_file = "module"
     readme_file = "README.md"
+    meta_file = "meta.yml"
     built_by_softpack_file = ".built_by_softpack"
     built_by_softpack = Type.softpack.value
     generated_from_module_file = ".generated_from_module"
@@ -154,6 +155,11 @@ def spec(self) -> Box:
                 map(lambda p: Package.from_name(p), info.packages)
             )
 
+            meta = Box()
+            if Artifacts.meta_file in self.obj:
+                meta = Box.from_yaml(self.obj[Artifacts.meta_file].data)
+            info["tags"] = getattr(meta, "tags", [])
+
             return info
 
         def __iter__(self) -> Iterator["Artifacts.Object"]:
@@ -322,7 +328,7 @@ def iter(self) -> Iterable:
 
         return itertools.chain.from_iterable(map(self.environments, folders))
 
-    def get(self, path: Path, name: str) -> Optional[pygit2.Tree]:
+    def get(self, path: Path, name: str) -> Optional[Object]:
         """Return the environment at the specified name and path.
 
         Args:
@@ -330,10 +336,13 @@ def get(self, path: Path, name: str) -> Optional[pygit2.Tree]:
             name: the name of the environment folder
 
         Returns:
-            pygit2.Tree: a pygit2.Tree or None
+            Object: an Object or None
         """
         try:
-            return self.tree(str(self.environments_folder(str(path), name)))
+            return self.Object(
+                Path(path, name),
+                self.tree(str(self.environments_folder(str(path), name))),
+            )
         except KeyError:
             return None
 
diff --git a/softpack_core/schemas/environment.py b/softpack_core/schemas/environment.py
index 5e0f9ae..8e989d8 100644
--- a/softpack_core/schemas/environment.py
+++ b/softpack_core/schemas/environment.py
@@ -11,7 +11,7 @@
 from dataclasses import dataclass
 from pathlib import Path
 from traceback import format_exception_only
-from typing import Iterable, List, Optional, Tuple, Union, cast
+from typing import List, Optional, Tuple, Union, cast
 
 import httpx
 import starlette.datastructures
@@ -52,6 +52,11 @@ class UpdateEnvironmentSuccess(Success):
     """Environment successfully updated."""
 
 
+@strawberry.type
+class AddTagSuccess(Success):
+    """Successfully added tag to environment."""
+
+
 @strawberry.type
 class DeleteEnvironmentSuccess(Success):
     """Environment successfully deleted."""
@@ -74,6 +79,7 @@ class EnvironmentNotFoundError(Error):
 
     path: str
     name: str
+    message: str = "No environment with this path and name found."
 
 
 @strawberry.type
@@ -109,6 +115,15 @@ class BuilderError(Error):
     ],
 )
 
+AddTagResponse = strawberry.union(
+    "AddTagResponse",
+    [
+        AddTagSuccess,
+        InvalidInputError,
+        EnvironmentNotFoundError,
+    ],
+)
+
 DeleteResponse = strawberry.union(
     "DeleteResponse",
     [
@@ -126,6 +141,32 @@ class BuilderError(Error):
 )
 
 
+def validate_tag(tag: str) -> Union[None, InvalidInputError]:
+    """If the given tag is invalid, return an error describing why, else None.
+
+    Tags must be composed solely of alphanumerics, dots, underscores,
+    dashes, and spaces, and not contain runs of multiple spaces or
+    leading/trailing whitespace.
+    """
+    if tag != tag.strip():
+        return InvalidInputError(
+            message="Tags must not contain leading or trailing whitespace"
+        )
+
+    if re.fullmatch(r"[a-zA-Z0-9 ._-]+", tag) is None:
+        return InvalidInputError(
+            message="Tags must contain only alphanumerics, dots, "
+            "underscores, dashes, and spaces"
+        )
+
+    if re.search(r"\s\s", tag) is not None:
+        return InvalidInputError(
+            message="Tags must not contain runs of multiple spaces"
+        )
+
+    return None
+
+
 @strawberry.input
 class PackageInput(Package):
     """A Strawberry input model representing a package."""
@@ -146,6 +187,7 @@ class EnvironmentInput:
     path: str
     description: str
     packages: list[PackageInput]
+    tags: Optional[list[str]] = None
 
     def validate(self) -> Union[None, InvalidInputError]:
         """Validate all values.
@@ -156,7 +198,11 @@ def validate(self) -> Union[None, InvalidInputError]:
         Returns:
             None if good, or InvalidInputError if not all values supplied.
         """
-        if any(len(value) == 0 for value in vars(self).values()):
+        if any(
+            len(value) == 0
+            for key, value in vars(self).items()
+            if key != "tags"
+        ):
             return InvalidInputError(message="all fields must be filled in")
 
         if not re.fullmatch("^[a-zA-Z0-9_-][a-zA-Z0-9_.-]*$", self.name):
@@ -178,6 +224,10 @@ def validate(self) -> Union[None, InvalidInputError]:
                 "alphanumerics, dash, and underscore"
             )
 
+        for tag in self.tags or []:
+            if (response := validate_tag(tag)) is not None:
+                return response
+
         return None
 
     @classmethod
@@ -255,6 +305,7 @@ class Environment:
     type: Type
     packages: list[Package]
     state: Optional[State]
+    tags: list[str]
     artifacts = Artifacts()
 
     requested: Optional[datetime.datetime] = None
@@ -263,7 +314,7 @@ class Environment:
     avg_wait_secs: Optional[float] = None
 
     @classmethod
-    def iter(cls) -> Iterable["Environment"]:
+    def iter(cls) -> list["Environment"]:
         """Get an iterator over all Environment objects.
 
         Returns:
@@ -323,6 +374,7 @@ def from_artifact(cls, obj: Artifacts.Object) -> Optional["Environment"]:
                 state=spec.state,
                 readme=spec.get("readme", ""),
                 type=spec.get("type", ""),
+                tags=spec.tags,
             )
         except KeyError:
             return None
@@ -443,7 +495,7 @@ def create_new_env(
                 name=env.name,
             )
 
-        # Create folder with place-holder file
+        # Create folder with initial files
         new_folder_path = Path(env.path, env.name)
         try:
             softpack_definition = dict(
@@ -453,13 +505,20 @@ def create_new_env(
                     for pkg in env.packages
                 ],
             )
-            ymlData = yaml.dump(softpack_definition)
+            definitionData = yaml.dump(softpack_definition)
+
+            meta = dict(tags=sorted(set(env.tags or [])))
+            metaData = yaml.dump(meta)
 
             tree_oid = cls.artifacts.create_files(
                 new_folder_path,
                 [
                     (env_type, ""),  # e.g. .built_by_softpack
-                    (cls.artifacts.environments_file, ymlData),  # softpack.yml
+                    (
+                        cls.artifacts.environments_file,
+                        definitionData,
+                    ),  # softpack.yml
+                    (cls.artifacts.meta_file, metaData),
                 ],
                 True,
             )
@@ -491,11 +550,52 @@ def check_env_exists(
             return None
 
         return EnvironmentNotFoundError(
-            message="No environment with this path and name found.",
             path=str(path.parent),
             name=path.name,
         )
 
+    @classmethod
+    def add_tag(
+        cls, name: str, path: str, tag: str
+    ) -> AddTagResponse:  # type: ignore
+        """Add a tag to an Environment.
+
+        Tags must be valid as defined by validate_tag().
+
+        Adding a tag that already exists is not an error.
+
+        Args:
+            name: the name of of environment
+            path: the path of the environment
+            tag: the tag to add
+
+        Returns:
+            A message confirming the success or failure of the operation.
+        """
+        environment_path = Path(path, name)
+        response: Optional[Error] = cls.check_env_exists(environment_path)
+        if response is not None:
+            return response
+
+        if (response := validate_tag(tag)) is not None:
+            return response
+
+        tree = cls.artifacts.get(Path(path), name)
+        if tree is None:
+            return EnvironmentNotFoundError(path=path, name=name)
+        box = tree.spec()
+        tags = set(box.tags)
+        if tag in tags:
+            return AddTagSuccess(message="Tag already present")
+        tags.add(tag)
+
+        metadata = yaml.dump({"tags": sorted(tags)})
+        tree_oid = cls.artifacts.create_file(
+            environment_path, cls.artifacts.meta_file, metadata, overwrite=True
+        )
+        cls.artifacts.commit_and_push(tree_oid, "create environment folder")
+        return AddTagSuccess(message="Tag successfully added")
+
     @classmethod
     def delete(cls, name: str, path: str) -> DeleteResponse:  # type: ignore
         """Delete an Environment.
@@ -749,6 +849,7 @@ class Mutation:
 
         createEnvironment: CreateResponse = Environment.create  # type: ignore
         deleteEnvironment: DeleteResponse = Environment.delete  # type: ignore
+        addTag: AddTagResponse = Environment.add_tag  # type: ignore
         # writeArtifact: WriteArtifactResponse = (  # type: ignore
         #     Environment.write_artifact
         # )
diff --git a/softpack_core/service.py b/softpack_core/service.py
index e9f11ab..ff38101 100644
--- a/softpack_core/service.py
+++ b/softpack_core/service.py
@@ -125,4 +125,8 @@ async def resend_pending_builds(  # type: ignore[no-untyped-def]
             response.status_code = 500
             message = "Failed to trigger all resends"
 
-        return {"message": message, "successes": successes, "failures": failures}
+        return {
+            "message": message,
+            "successes": successes,
+            "failures": failures,
+        }
diff --git a/tests/integration/test_builderupload.py b/tests/integration/test_builderupload.py
index b250177..a752b43 100644
--- a/tests/integration/test_builderupload.py
+++ b/tests/integration/test_builderupload.py
@@ -54,5 +54,5 @@ def test_builder_upload(testable_env_input):
     tree = Environment.artifacts.get(env_parent, env_name)
     assert tree is not None
 
-    assert tree[softpackYaml].data == softpackYamlContents
-    assert tree[spackLock].data == spackLockContents
+    assert tree.get(softpackYaml).data == softpackYamlContents
+    assert tree.get(spackLock).data == spackLockContents
diff --git a/tests/integration/test_environment.py b/tests/integration/test_environment.py
index 478fd9c..d59fb5d 100644
--- a/tests/integration/test_environment.py
+++ b/tests/integration/test_environment.py
@@ -1,4 +1,4 @@
-"""Copyright (c) 2023 Genome Research Ltd.
+"""Copyright (c) 2023, 2024 Genome Research Ltd.
 
 This source code is licensed under the MIT license found in the
 LICENSE file in the root directory of this source tree.
@@ -16,6 +16,7 @@
 
 from softpack_core.artifacts import Artifacts
 from softpack_core.schemas.environment import (
+    AddTagSuccess,
     BuilderError,
     CreateEnvironmentSuccess,
     DeleteEnvironmentSuccess,
@@ -51,6 +52,7 @@ def test_create(httpx_post, testable_env_input: EnvironmentInput) -> None:
                 Package(name="pkg_test2"),
                 Package(name="pkg_test3", version="3.1"),
             ],
+            tags=["foo", "foo", "bar"],
         )
     )
     assert isinstance(result, CreateEnvironmentSuccess)
@@ -70,6 +72,10 @@ def test_create(httpx_post, testable_env_input: EnvironmentInput) -> None:
         "packages": ["pkg_test"],
     }
     assert yaml.safe_load(ymlFile.data.decode()) == expected_yaml
+    meta_yml = file_in_remote(dir / Environment.artifacts.meta_file)
+    expected_meta_yml = {"tags": []}
+    actual_meta_yml = yaml.safe_load(meta_yml.data.decode())
+    assert actual_meta_yml == expected_meta_yml
 
     dir = Path(
         Environment.artifacts.environments_root,
@@ -86,6 +92,11 @@ def test_create(httpx_post, testable_env_input: EnvironmentInput) -> None:
     }
     assert yaml.safe_load(ymlFile.data.decode()) == expected_yaml
 
+    meta_yml = file_in_remote(dir / Environment.artifacts.meta_file)
+    expected_meta_yml = {"tags": ["bar", "foo"]}
+    actual_meta_yml = yaml.safe_load(meta_yml.data.decode())
+    assert actual_meta_yml == expected_meta_yml
+
     result = Environment.create(testable_env_input)
     testable_env_input.name = orig_input_name
     assert isinstance(result, CreateEnvironmentSuccess)
@@ -99,6 +110,19 @@ def test_create(httpx_post, testable_env_input: EnvironmentInput) -> None:
     assert file_in_remote(path)
 
 
+def test_create_no_tags(httpx_post, testable_env_input):
+    testable_env_input.tags = None
+    result = Environment.create(testable_env_input)
+    assert isinstance(result, CreateEnvironmentSuccess)
+
+
+def test_create_illegal_tags(httpx_post, testable_env_input):
+    for tag in ["   ", " ", " leading whitespace", "trailing whitespace "]:
+        testable_env_input.tags = [tag]
+        result = Environment.create(testable_env_input)
+        assert isinstance(result, InvalidInputError)
+
+
 def test_create_name_empty_disallowed(httpx_post, testable_env_input):
     testable_env_input.name = ""
     result = Environment.create(testable_env_input)
@@ -469,6 +493,7 @@ async def test_create_from_module(httpx_post, testable_env_input):
     )
     assert isinstance(result, EnvironmentNotFoundError)
 
+
 def test_environmentinput_from_path():
     for path in (
         "users/any1/envName",
@@ -485,4 +510,46 @@ def test_environmentinput_from_path():
         "users/any1/..envName-1",
         "users/any1/../envName-1.1",
     ]:
-        assert EnvironmentInput.from_path(path).validate() is not None
\ No newline at end of file
+        assert EnvironmentInput.from_path(path).validate() is not None
+
+
+def test_tagging(httpx_post, testable_env_input: EnvironmentInput) -> None:
+    example_env = Environment.iter()[0]
+    assert example_env.tags == []
+
+    name, path = example_env.name, example_env.path
+    result = Environment.add_tag(name, path, tag="test")
+    assert isinstance(result, AddTagSuccess)
+    assert result.message == "Tag successfully added"
+
+    result = Environment.add_tag("foo", "users/xyz", tag="test")
+    assert isinstance(result, EnvironmentNotFoundError)
+
+    result = Environment.add_tag(name, path, tag="../")
+    assert isinstance(result, InvalidInputError)
+
+    result = Environment.add_tag(name, path, tag="")
+    assert isinstance(result, InvalidInputError)
+
+    result = Environment.add_tag(name, path, tag="         ")
+    assert isinstance(result, InvalidInputError)
+
+    result = Environment.add_tag(name, path, tag="foo  bar")
+    assert isinstance(result, InvalidInputError)
+
+    example_env = Environment.iter()[0]
+    assert len(example_env.tags) == 1
+    assert example_env.tags[0] == "test"
+
+    result = Environment.add_tag(name, path, tag="second test")
+    assert isinstance(result, AddTagSuccess)
+
+    example_env = Environment.iter()[0]
+    assert example_env.tags == ["second test", "test"]
+
+    result = Environment.add_tag(name, path, tag="test")
+    assert isinstance(result, AddTagSuccess)
+    assert result.message == "Tag already present"
+
+    example_env = Environment.iter()[0]
+    assert example_env.tags == ["second test", "test"]