diff --git a/changelog/1.feature.md b/changelog/2.feature.md
similarity index 100%
rename from changelog/1.feature.md
rename to changelog/2.feature.md
diff --git a/changelog/4.feature.md b/changelog/4.feature.md
new file mode 100644
index 0000000..504f5af
--- /dev/null
+++ b/changelog/4.feature.md
@@ -0,0 +1,2 @@
+Adds the placeholder concept of `Executor`'s which are responsible for running metrics
+in different environments.
diff --git a/docs/cli/.gitkeep b/docs/cli/.gitkeep
deleted file mode 100644
index e69de29..0000000
diff --git a/docs/configuration.md b/docs/configuration.md
new file mode 100644
index 0000000..938e0bb
--- /dev/null
+++ b/docs/configuration.md
@@ -0,0 +1,14 @@
+# Configuration
+
+## Environment Variables
+
+Environment variables are used to control some aspects of the model.
+The default values for these environment variables are generally suitable,
+but if you require updating these values we recommend the use of a `.env` file
+to make the changes easier to reproduce in future.
+
+### `CMIP_REF_EXECUTOR`
+
+Executor to use for running the metrics.
+
+Defaults to use the local executor ("local").
diff --git a/docs/explanation.md b/docs/explanation.md
index 9da03cb..ba4e8f3 100644
--- a/docs/explanation.md
+++ b/docs/explanation.md
@@ -11,3 +11,21 @@ Points we will aim to cover:
 
 We will aim to avoid writing instructions or technical descriptions here,
 they belong elsewhere.
+
+
+## Execution Environments
+
+The REF aims to support the execution of metrics in a variety of environments.
+This includes local execution, testing, cloud-based execution, and execution on HPC systems.
+
+The currently supported execution environments are:
+
+* Local
+
+The following environments are planned to be supported in the future:
+
+* Kubernetes (for cloud-based execution)
+* Subprocess (for HPC systems)
+
+The selected executor is defined using the `CMIP_REF_EXECUTOR` environment variable.
+See the [Configuration](configuration.md) page for more information.
diff --git a/docs/gen_doc_stubs.py b/docs/gen_doc_stubs.py
index bfd991a..0d7cefb 100644
--- a/docs/gen_doc_stubs.py
+++ b/docs/gen_doc_stubs.py
@@ -118,11 +118,14 @@ def write_module_page(
         fh.write("\n")
         fh.write(f"::: {package_full_name}")
 
-    package_doc_split = package.__doc__.splitlines()
-    if not package_doc_split[0]:
-        summary = package_doc_split[1]
+    if package.__doc__ is None:
+        summary = ""
     else:
-        summary = package_doc_split[0]
+        package_doc_split = package.__doc__.splitlines()
+        if not package_doc_split[0]:
+            summary = package_doc_split[1]
+        else:
+            summary = package_doc_split[0]
 
     return PackageInfo(package_full_name, package_name, summary)
 
diff --git a/mkdocs.yml b/mkdocs.yml
index 3db6ae5..b0461d2 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -9,6 +9,7 @@ repo_url: https://github.com/CMIP-REF/cmip-ref
 nav:
   - CMIP REF: index.md
   - Installation: installation.md
+  - Configuration: configuration.md
   - How-to guides:
     - how-to-guides/index.md
   - Tutorials: tutorials.md
diff --git a/packages/ref-core/src/ref_core/executor/__init__.py b/packages/ref-core/src/ref_core/executor/__init__.py
new file mode 100644
index 0000000..1fd0235
--- /dev/null
+++ b/packages/ref-core/src/ref_core/executor/__init__.py
@@ -0,0 +1,132 @@
+"""
+Execute metrics in different environments
+
+We support running metrics in different environments, such as locally,
+in a separate process, or in a container.
+These environments are represented by `Executor` classes.
+The `CMIP_REF_EXECUTOR` environment variable determines which executor is used.
+
+The simplest executor is the `LocalExecutor`, which runs the metric in the same process.
+This is useful for local testing and debugging.
+
+This is a placeholder implementation and will be expanded in the future.
+"""
+
+import os
+from typing import Protocol, runtime_checkable
+
+from .local import LocalExecutor
+
+
+@runtime_checkable
+class Executor(Protocol):
+    """
+    An executor is responsible for running a metric.
+
+    The metric may be run locally in the same process or in a separate process or container.
+
+    Notes
+    -----
+    This is an extremely basic interface and will be expanded in the future, as we figure out
+    our requirements.
+    """
+
+    name: str
+
+    def run_metric(self, metric: object, *args, **kwargs) -> object:  # type: ignore
+        """
+        Execute a metric
+        """
+        # TODO: Add type hints for metric and return value in follow-up PR
+        ...
+
+
+class ExecutorManager:
+    """
+    Enables the registration of executors and retrieval by name.
+
+    This is exposed as a singleton instance `ref_core.executor.get_executor`
+     and `ref_core.executor.register_executor`,
+     but for testability, you can create your own instance.
+    """
+
+    def __init__(self) -> None:
+        self._executors: dict[str, Executor] = {}
+
+    def register(self, executor: Executor) -> None:
+        """
+        Register an executor with the manager
+
+        Parameters
+        ----------
+        executor
+            The executor to register
+        """
+        if not isinstance(executor, Executor):  # pragma: no cover
+            raise ValueError("Executor must be an instance of Executor")
+        self._executors[executor.name.lower()] = executor
+
+    def get(self, name: str) -> Executor:
+        """
+        Get an executor by name
+
+        Parameters
+        ----------
+        name
+            Name of the executor (case-sensitive)
+
+        Raises
+        ------
+        KeyError
+            If the executor with the given name is not found
+
+        Returns
+        -------
+        :
+            The requested executor
+        """
+        return self._executors[name.lower()]
+
+
+_default_manager = ExecutorManager()
+
+register_executor = _default_manager.register
+get_executor = _default_manager.get
+
+
+def run_metric(metric_name: str, *args, **kwargs) -> object:  # type: ignore
+    """
+    Run a metric using the default executor
+
+    The executor is determined by the `CMIP_REF_EXECUTOR` environment variable.
+    The arguments will be updated in the future as the metric execution interface is expanded.
+
+    TODO: migrate to a configuration object rather than relying on environment variables.
+
+    Parameters
+    ----------
+    metric_name
+        Name of the metric to run.
+
+        Eventually the metric will be sourced from via some kind of registry.
+        For now, it's just a placeholder.
+    args
+        Extra arguments passed to the metric of interest
+    kwargs
+        Extra keyword arguments passed to the metric of interest
+
+    Returns
+    -------
+    :
+        The result of the metric execution
+    """
+    executor_name = os.environ.get("CMIP_REF_EXECUTOR", "local")
+
+    executor = get_executor(executor_name)
+    # metric = get_metric(metric_name)  # TODO: Implement this
+    metric = kwargs.pop("metric")
+
+    return executor.run_metric(metric, *args, **kwargs)
+
+
+register_executor(LocalExecutor())
diff --git a/packages/ref-core/src/ref_core/executor/local.py b/packages/ref-core/src/ref_core/executor/local.py
new file mode 100644
index 0000000..af48231
--- /dev/null
+++ b/packages/ref-core/src/ref_core/executor/local.py
@@ -0,0 +1,27 @@
+class LocalExecutor:
+    """
+    Run a metric locally, in-process.
+
+    This is mainly useful for debugging and testing.
+    The production executor will run the metric in a separate process or container,
+    the exact manner of which is yet to be determined.
+    """
+
+    name = "local"
+
+    def run_metric(self, metric, *args, **kwargs):  # type: ignore
+        """
+        Run a metric in process
+
+        Parameters
+        ----------
+        metric
+        args
+        kwargs
+
+        Returns
+        -------
+        :
+            Results from running the metric
+        """
+        return metric.run(*args, **kwargs)
diff --git a/packages/ref-core/tests/unit/test_executor.py b/packages/ref-core/tests/unit/test_executor.py
new file mode 100644
index 0000000..ceaa3d9
--- /dev/null
+++ b/packages/ref-core/tests/unit/test_executor.py
@@ -0,0 +1,59 @@
+import pytest
+from ref_core.executor import Executor, ExecutorManager, run_metric
+from ref_core.executor.local import LocalExecutor
+
+
+class MockMetric:
+    def run(self, *args, **kwargs):
+        result = {
+            "args": args,
+            "kwargs": kwargs,
+        }
+
+        return result
+
+
+class TestExecutorManager:
+    def test_executor_register(self):
+        manager = ExecutorManager()
+        manager.register(LocalExecutor())
+
+        assert len(manager._executors) == 1
+        assert "local" in manager._executors
+        assert isinstance(manager.get("local"), LocalExecutor)
+
+
+class TestLocalExecutor:
+    def test_is_executor(self):
+        executor = LocalExecutor()
+
+        assert executor.name == "local"
+        assert isinstance(executor, Executor)
+
+    def test_run_metric(self):
+        executor = LocalExecutor()
+
+        metric = MockMetric()
+        result = executor.run_metric(metric, "test", kwarg="test")
+
+        assert result == {
+            "args": ("test",),
+            "kwargs": {"kwarg": "test"},
+        }
+
+
+@pytest.mark.parametrize("executor_name", ["local", None])
+def test_run_metric_local(monkeypatch, executor_name):
+    if executor_name:
+        monkeypatch.setenv("CMIP_REF_EXECUTOR", executor_name)
+    result = run_metric("example_metric", "test", kwarg="test", metric=MockMetric())
+    assert result == {
+        "args": ("test",),
+        "kwargs": {"kwarg": "test"},
+    }
+
+
+def test_run_metric_unknown(monkeypatch):
+    monkeypatch.setenv("CMIP_REF_EXECUTOR", "missing")
+    with pytest.raises(KeyError):
+        run_metric("anything", "test", kwarg="test")