diff --git a/ctis/__init__.py b/ctis/__init__.py
index 41d44a5..b8c9e63 100644
--- a/ctis/__init__.py
+++ b/ctis/__init__.py
@@ -2,3 +2,9 @@
 A package for inverting imagery captured by a computed tomography imaging
 spectrograph.
 """
+
+from . import instruments
+
+__all__ = [
+    "instruments",
+]
diff --git a/ctis/instruments/__init__.py b/ctis/instruments/__init__.py
new file mode 100644
index 0000000..cd2312f
--- /dev/null
+++ b/ctis/instruments/__init__.py
@@ -0,0 +1,10 @@
+"""
+Models of CTIS instruments used during inversions.
+"""
+
+from ._instruments import AbstractInstrument, Instrument
+
+__all__ = [
+    "AbstractInstrument",
+    "Instrument",
+]
diff --git a/ctis/instruments/_instruments.py b/ctis/instruments/_instruments.py
new file mode 100644
index 0000000..7b059ad
--- /dev/null
+++ b/ctis/instruments/_instruments.py
@@ -0,0 +1,85 @@
+from typing import Callable
+import abc
+import dataclasses
+import named_arrays as na
+
+__all__ = [
+    "AbstractInstrument",
+    "Instrument",
+]
+
+
+ProjectionCallable = Callable[
+    [na.FunctionArray[na.SpectralPositionalVectorArray, na.ScalarArray]],
+    na.FunctionArray[na.SpectralPositionalVectorArray, na.ScalarArray],
+]
+
+
+@dataclasses.dataclass
+class AbstractInstrument(
+    abc.ABC,
+):
+    """
+    An interface describing a CTIS instrument.
+
+    This consists of a forward model
+    (which maps the spectral radiance of a physical scene to counts on a detector)
+    and a deprojection model
+    (which maps detector counts to the spectral radiance of a physical scene).
+    """
+
+    @abc.abstractmethod
+    def project(
+        self,
+        scene: na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar],
+    ) -> na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar]:
+        """
+        The forward model of the CTIS instrument.
+        Maps spectral and spatial coordinates on the field to coordinates
+        on the detector.
+
+        Parameters
+        ----------
+        scene
+            The spectral radiance of each spatial/spectral point in the scene.
+        """
+
+    @abc.abstractmethod
+    def deproject(
+        self,
+        projections: na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar],
+    ) -> ProjectionCallable:
+        """
+        The deprojection model of the CTIS instrument.
+        Maps spectral and spatial coordinates on the detector to coordinates
+        on the field.
+
+        Parameters
+        ----------
+        projections
+            The counts gathered by each detector in the CTIS instrument.
+        """
+
+
+@dataclasses.dataclass
+class Instrument(
+    AbstractInstrument,
+):
+    """
+    A CTIS instrument where the forward and deprojection models are explicitly
+    provided.
+    """
+
+    project: ProjectionCallable = dataclasses.MISSING
+    """
+    The forward model of the CTIS instrument.
+    Maps spectral and spatial coordinates on the field to coordinates
+    on the detector.
+    """
+
+    deproject: ProjectionCallable = dataclasses.MISSING
+    """
+    The deprojection model of the CTIS instrument.
+    Maps spectral and spatial coordinates on the detector to coordinates
+    on the field.
+    """
diff --git a/ctis/instruments/_instruments_test.py b/ctis/instruments/_instruments_test.py
new file mode 100644
index 0000000..7c6480d
--- /dev/null
+++ b/ctis/instruments/_instruments_test.py
@@ -0,0 +1,30 @@
+import pytest
+import abc
+import ctis
+
+
+class AbstractTestAbstractInstrument(
+    abc.ABC,
+):
+    def test_project(self, a: ctis.instruments.AbstractInstrument):
+        result = a.project
+        assert hasattr(result, "__call__")
+
+    def test_deproject(self, a: ctis.instruments.AbstractInstrument):
+        result = a.deproject
+        assert hasattr(result, "__call__")
+
+
+@pytest.mark.parametrize(
+    argnames="a",
+    argvalues=[
+        ctis.instruments.Instrument(
+            project=lambda x: x,
+            deproject=lambda x: x,
+        )
+    ],
+)
+class TestInstrument(
+    AbstractTestAbstractInstrument,
+):
+    pass