diff --git a/ctis/__init__.py b/ctis/__init__.py
index 8107466..fe5117f 100644
--- a/ctis/__init__.py
+++ b/ctis/__init__.py
@@ -4,7 +4,9 @@
 """
 
 from . import scenes
+from . import instruments
 
 __all__ = [
     "scenes",
+    "instruments",
 ]
diff --git a/ctis/instruments/__init__.py b/ctis/instruments/__init__.py
new file mode 100644
index 0000000..bcc5400
--- /dev/null
+++ b/ctis/instruments/__init__.py
@@ -0,0 +1,15 @@
+"""
+Models of CTIS instruments used during inversions.
+"""
+
+from ._instruments import (
+    AbstractInstrument,
+    AbstractLinearInstrument,
+    IdealInstrument,
+)
+
+__all__ = [
+    "AbstractInstrument",
+    "AbstractLinearInstrument",
+    "IdealInstrument",
+]
diff --git a/ctis/instruments/_instruments.py b/ctis/instruments/_instruments.py
new file mode 100644
index 0000000..92ad102
--- /dev/null
+++ b/ctis/instruments/_instruments.py
@@ -0,0 +1,107 @@
+from typing import Callable, Sequence
+import abc
+import dataclasses
+import astropy.units as u
+import named_arrays as na
+
+__all__ = [
+    "AbstractInstrument",
+    "AbstractLinearInstrument",
+    "IdealInstrument",
+]
+
+
+ProjectionCallable = Callable[
+    [na.FunctionArray[na.SpectralPositionalVectorArray, na.ScalarArray]],
+    na.FunctionArray[na.SpectralPositionalVectorArray, na.ScalarArray],
+]
+
+
+@dataclasses.dataclass
+class AbstractInstrument(
+    abc.ABC,
+):
+    """
+    An interface describing a general CTIS instrument.
+
+    The only member of this interface is :meth:`image`,
+    which represents the forward model of the 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 image(
+        self,
+        scene: na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar],
+    ) -> na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar]:
+        f"""
+        The forward model of this CTIS instrument, which maps spectral radiance
+        on the skyplane to counts on the detectors.
+        
+        Parameters
+        ----------
+        scene
+            The spectral radiance in units equivalent to 
+            {(u.erg / (u.cm**2 * u.sr * u.AA * u.s)):latex_inline}.
+        """
+
+    @property
+    @abc.abstractmethod
+    def coordinates_scene(self) -> na.AbstractSpectralPositionalVectorArray:
+        """
+        A grid of wavelength and position coordinates on the skyplane
+        which will be used to construct the inverted scene.
+
+        Normally the pitch of this grid is chosen to be the average
+        plate scale of the instrument.
+        """
+
+
+@dataclasses.dataclass
+class AbstractLinearInstrument(
+    AbstractInstrument,
+):
+    """
+    An instrument that can be modeled using matrix multiplication.
+    """
+
+    @property
+    @abc.abstractmethod
+    def _weights(self) -> tuple[na.AbstractScalar, dict[str, int], dict[str, int]]:
+        """
+        A sparse matrix which maps spectral radiance on the skyplane to
+        the number of electrons measured by the sensor.
+        """
+
+    def image(
+        self,
+        scene: na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar],
+    ) -> na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar]:
+        pass
+
+    def project(
+        self,
+        scene: na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar],
+    ) -> na.FunctionArray[na.SpectralPositionalVectorArray, na.AbstractScalar]:
+
+        pass
+
+
+@dataclasses.dataclass
+class IdealInstrument(
+    AbstractInstrument,
+):
+    """
+    An idealized CTIS instrument which has a perfect point-spread function
+    and no noise.
+    """
+
+    dispersion: u.Quantity | na.AbstractScalar
+    r"""The magnitude of the dispersion in :math:`\text{m \AA} \,\text{pix}^-1`"""
+
+    angle: u.Quantity | na.AbstractScalar
+    """The angle of the dispersion direction with respect to the scene."""
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