diff --git a/src/galax/dynamics/__init__.py b/src/galax/dynamics/__init__.py
index 64ee8788..e5e74abe 100644
--- a/src/galax/dynamics/__init__.py
+++ b/src/galax/dynamics/__init__.py
@@ -6,6 +6,7 @@
     "integrate",
     "mockstream",
     "plot",
+    "cluster",
     # integrate
     "evaluate_orbit",
     "Orbit",
@@ -29,7 +30,7 @@
 from galax.setup_package import RUNTIME_TYPECHECKER
 
 with install_import_hook("galax.dynamics", RUNTIME_TYPECHECKER):
-    from . import fields, integrate, mockstream, plot
+    from . import cluster, fields, integrate, mockstream, plot
     from ._src.cluster.funcs import lagrange_points, tidal_radius
     from ._src.funcs import specific_angular_momentum
     from ._src.orbit import Orbit
diff --git a/src/galax/dynamics/_src/cluster/__init__.py b/src/galax/dynamics/_src/cluster/__init__.py
index 7f466a23..1a33955c 100644
--- a/src/galax/dynamics/_src/cluster/__init__.py
+++ b/src/galax/dynamics/_src/cluster/__init__.py
@@ -1,9 +1,25 @@
-"""Cluster mass evolution."""
+"""Cluster evolution."""
 
 __all__ = [
+    "MassSolver",
+    # Fields
+    "MassVectorField",
+    "AbstractMassField",
+    "UserMassField",
+    "ConstantMassField",
+    # Events
+    "MassBelowThreshold",
+    # Functions
     "lagrange_points",
     "tidal_radius",
 ]
 
-
+from .events import MassBelowThreshold
+from .fields import (
+    AbstractMassField,
+    ConstantMassField,
+    MassVectorField,
+    UserMassField,
+)
 from .funcs import lagrange_points, tidal_radius
+from .solver import MassSolver
diff --git a/src/galax/dynamics/_src/cluster/events.py b/src/galax/dynamics/_src/cluster/events.py
new file mode 100644
index 00000000..f6c1a31a
--- /dev/null
+++ b/src/galax/dynamics/_src/cluster/events.py
@@ -0,0 +1,67 @@
+"""``galax`` dynamics."""
+
+__all__ = ["MassBelowThreshold"]
+
+from typing import Any
+
+import equinox as eqx
+from jaxtyping import Array
+from plum import dispatch
+
+import unxt as u
+from unxt.quantity import AbstractQuantity
+
+
+class MassBelowThreshold(eqx.Module):  # type: ignore[misc]
+    """Event to stop integration when the mass falls below a threshold.
+
+    Instances can be used as the ``cond_fn`` argument of `diffrax.Event`. Since
+    this returns a scalar (not a `bool`) the solve the solve will terminate on
+    the step when the mass is below the threshold.
+
+    With `diffrax.Event` this can be combined with a root-finder to find the
+    exact time when the mass is below the threshold, rather than the step
+    after.
+
+    Example
+    -------
+    >>> import unxt as u
+    >>> from galax.dynamics.cluster import MassBelowThreshold
+
+    >>> cond_fn = MassBelowThreshold(u.Quantity(0.0, "Msun"))
+    >>> args = {"units": u.unitsystems.galactic}
+
+    >>> cond_fn(0.0, u.Quantity(1.0, "Msun"), args)
+    Array(1., dtype=float64, weak_type=True)
+
+    >>> cond_fn(0.0, u.Quantity(0.0, "Msun"), args)
+    Array(0., dtype=float64, weak_type=True)
+
+    TODO: example using it as a with `diffrax.Event`.
+
+    """
+
+    #: Threshold mass at which to stop integration.
+    threshold: AbstractQuantity
+
+    @dispatch
+    def __call__(
+        self: "MassBelowThreshold",
+        t: Any,  # noqa: ARG002
+        y: Array,
+        args: dict[str, Any],
+        /,
+        **kwargs: Any,  # noqa: ARG002
+    ) -> Array:
+        return y - self.threshold.ustrip(args["units"])
+
+    @dispatch
+    def __call__(
+        self: "MassBelowThreshold",
+        t: Any,  # noqa: ARG002
+        y: AbstractQuantity,
+        args: dict[str, Any],
+        /,
+        **kwargs: Any,  # noqa: ARG002
+    ) -> Array:
+        return u.ustrip(args["units"], y - self.threshold)
diff --git a/src/galax/dynamics/_src/cluster/fields.py b/src/galax/dynamics/_src/cluster/fields.py
new file mode 100644
index 00000000..51df95f0
--- /dev/null
+++ b/src/galax/dynamics/_src/cluster/fields.py
@@ -0,0 +1,122 @@
+"""Fields for mass evolution."""
+
+__all__ = [
+    "MassVectorField",
+    "AbstractMassField",
+    "UserMassField",
+    "ConstantMassField",
+]
+
+from abc import abstractmethod
+from dataclasses import KW_ONLY
+from typing import Any, Protocol, TypeAlias, TypedDict, runtime_checkable
+
+import diffrax as dfx
+import equinox as eqx
+import jax.numpy as jnp
+from jaxtyping import Array, PyTree
+
+from galax.dynamics._src.fields import AbstractField
+
+Time: TypeAlias = Any
+ClusterMass: TypeAlias = Any
+
+
+class Args(TypedDict, total=False):
+    units: Any
+    # Add other optional keys here if needed
+
+
+@runtime_checkable
+class MassVectorField(Protocol):
+    """Protocol for mass vector field.
+
+    This is a function that returns the derivative of the mass vector with
+    respect to time.
+
+    Examples
+    --------
+    >>> from galax.dynamics.cluster import MassVectorField
+
+    >>> def mass_deriv(t, Mc, args, **kwargs): pass
+
+    >>> isinstance(mass_deriv, MassVectorField)
+    True
+
+    """
+
+    def __call__(
+        self, t: Time, Mc: ClusterMass, args: Args, /, **kwargs: Any
+    ) -> Array: ...
+
+
+class AbstractMassField(AbstractField):
+    """ABC for mass fields.
+
+    Methods
+    -------
+    __call__ : `galax.dynamics.cluster.MassVectorField`
+        Compute the mass field.
+    terms : the `diffrax.AbstractTerm` `jaxtyping.PyTree` for integration.
+
+    """
+
+    @abstractmethod
+    def __call__(self, t: Time, Mc: ClusterMass, args: Args, /, **kwargs: Any) -> Array:  # type: ignore[override]
+        raise NotImplementedError  # pragma: no cover
+
+    @AbstractField.terms.dispatch  # type: ignore[misc]
+    def terms(
+        self: "AbstractMassField", _: dfx.AbstractSolver, /
+    ) -> PyTree[dfx.AbstractTerm]:
+        """Return diffeq terms for integration.
+
+        Examples
+        --------
+        >>> import diffrax as dfx
+        >>> import galax.dynamics as gd
+
+        >>> field = gd.cluster.ConstantMassField()
+        >>> field.terms(dfx.Dopri8())
+        ODETerm(
+            vector_field=_JitWrapper( fn='ConstantMassField.__call__', ... ) )
+
+        """
+        return dfx.ODETerm(eqx.filter_jit(self.__call__))
+
+
+#####################################################
+
+
+class UserMassField(AbstractMassField):
+    """User-defined mass field.
+
+    This takes a user-defined function of type
+    `galax.dynamics.cluster.MassVectorField`.
+
+    """
+
+    #: User-defined mass derivative function of type
+    #: `galax.dynamics.cluster.MassVectorField`
+    mass_deriv: MassVectorField
+
+    _: KW_ONLY
+
+    def __call__(self, t: Time, Mc: ClusterMass, args: Args, /, **kwargs: Any) -> Array:  # type: ignore[override]
+        return self.mass_deriv(t, Mc, args, **kwargs)
+
+
+#####################################################
+
+
+class ConstantMassField(AbstractMassField):
+    """Constant mass field.
+
+    This is a constant mass field.
+
+
+
+    """
+
+    def __call__(self, t: Time, Mc: ClusterMass, args: Args, /, **kwargs: Any) -> Array:  # type: ignore[override]  # noqa: ARG002
+        return jnp.zeros_like(Mc)
diff --git a/src/galax/dynamics/_src/cluster/solver.py b/src/galax/dynamics/_src/cluster/solver.py
new file mode 100644
index 00000000..f4be8929
--- /dev/null
+++ b/src/galax/dynamics/_src/cluster/solver.py
@@ -0,0 +1,127 @@
+"""``galax`` dynamics."""
+
+__all__ = ["MassSolver"]
+
+from dataclasses import KW_ONLY
+from typing import Any
+
+import diffrax as dfx
+import equinox as eqx
+import optimistix as optx
+from plum import dispatch
+
+import unxt as u
+from unxt.quantity import AbstractQuantity
+
+from .events import MassBelowThreshold
+from .fields import AbstractMassField
+from galax.dynamics._src.diffeq import DiffEqSolver
+from galax.dynamics._src.solver import AbstractSolver
+from galax.dynamics._src.utils import parse_saveat
+
+
+class MassSolver(AbstractSolver, strict=True):  # type: ignore[call-arg]
+    """Solver for mass history."""
+
+    diffeqsolver: DiffEqSolver = eqx.field(
+        default=DiffEqSolver(
+            solver=dfx.Dopri8(),
+            stepsize_controller=dfx.PIDController(rtol=1e-8, atol=1e-8),
+        ),
+        converter=DiffEqSolver.from_,
+    )
+    # TODO: should this be incorporated into in `DiffEqSolver`?
+    event: dfx.Event = eqx.field(
+        default=dfx.Event(
+            cond_fn=MassBelowThreshold(u.Quantity(0.0, "Msun")),
+            root_finder=optx.Newton(1e-5, 1e-5, optx.rms_norm),
+        )
+    )
+
+    _: KW_ONLY
+
+    units: u.AbstractUnitSystem = eqx.field(
+        default=u.unitsystems.galactic, converter=u.unitsystem, static=True
+    )
+
+    @dispatch.abstract
+    def init(
+        self: "MassSolver", terms: Any, t0: Any, t1: Any, y0: Any, args: Any
+    ) -> Any:
+        # See dispatches below
+        raise NotImplementedError  # pragma: no cover
+
+    @dispatch.abstract
+    def step(
+        self: "MassSolver",
+        terms: Any,
+        t0: Any,
+        t1: Any,
+        y0: Any,
+        args: Any,
+        **step_kwargs: Any,  # e.g. solver_state, made_jump
+    ) -> Any:
+        """Step the state."""
+        # See dispatches below
+        raise NotImplementedError  # pragma: no cover
+
+    # TODO: dispatch where the state from `init` is accepted
+    @dispatch.abstract
+    def solve(
+        self: "MassSolver",
+        field: Any,
+        state: Any,
+        t0: Any,
+        t1: Any,
+        /,
+        args: Any = (),
+        **solver_kw: Any,  # TODO: TypedDict
+    ) -> dfx.Solution:
+        """Call `diffrax.diffeqsolve`."""
+        raise NotImplementedError  # pragma: no cover
+
+
+# ===================================================================
+# Solve Dispatches
+
+
+default_saveat = dfx.SaveAt(t1=True)
+
+
+@MassSolver.solve.dispatch  # type: ignore[misc]
+@eqx.filter_jit  # type: ignore[misc]
+def solve(
+    self: MassSolver,
+    field: AbstractMassField,
+    state: Any,
+    t0: AbstractQuantity,
+    t1: AbstractQuantity,
+    /,
+    saveat: Any = default_saveat,
+    **solver_kw: Any,
+) -> dfx.Solution:
+    # Units
+    units = self.units
+    utime = units["time"]
+
+    # Initial conditions
+    y0 = state.ustrip(units["mass"])  # Mc
+
+    # Extra info
+    args = {"units": units}
+
+    # Solve the differential equation
+    solver_kw.setdefault("dt0", None)
+    saveat = parse_saveat(units, saveat, dense=solver_kw.pop("dense", None))
+    soln = self.diffeqsolver(
+        field.terms(self.diffeqsolver),
+        t0=t0.ustrip(utime),
+        t1=t1.ustrip(utime),
+        y0=y0,
+        event=self.event,
+        args=args,
+        saveat=saveat,
+        **solver_kw,
+    )
+
+    return soln  # noqa: RET504
diff --git a/src/galax/dynamics/_src/fields.py b/src/galax/dynamics/_src/fields.py
index 82475f06..b27a9c8a 100644
--- a/src/galax/dynamics/_src/fields.py
+++ b/src/galax/dynamics/_src/fields.py
@@ -10,17 +10,12 @@
 from jaxtyping import PyTree
 from plum import dispatch
 
-import unxt as u
-
 from galax.dynamics._src.diffeq import DiffEqSolver
 
 
 class AbstractField(eqx.Module, strict=True):  # type: ignore[misc,call-arg]
     """Abstract base class for fields."""
 
-    #: unit system of the field.
-    units: eqx.AbstractVar[u.AbstractUnitSystem]
-
     @abstractmethod
     def __call__(self, t: Any, *args: Any, **kwargs: Any) -> Any:
         """Evaluate the field at time `t`."""
diff --git a/src/galax/dynamics/cluster.py b/src/galax/dynamics/cluster.py
new file mode 100644
index 00000000..7b287f00
--- /dev/null
+++ b/src/galax/dynamics/cluster.py
@@ -0,0 +1,34 @@
+""":mod:`galax.dynamics.cluster`."""
+
+__all__ = [
+    "MassSolver",
+    # Fields
+    "MassVectorField",
+    "AbstractMassField",
+    "UserMassField",
+    "ConstantMassField",
+    # Events
+    "MassBelowThreshold",
+    # Functions
+    "lagrange_points",
+    "tidal_radius",
+]
+
+from jaxtyping import install_import_hook
+
+from galax.setup_package import RUNTIME_TYPECHECKER
+
+with install_import_hook("galax.dynamics.fields", RUNTIME_TYPECHECKER):
+    from ._src.cluster import (
+        AbstractMassField,
+        ConstantMassField,
+        MassBelowThreshold,
+        MassSolver,
+        MassVectorField,
+        UserMassField,
+        lagrange_points,
+        tidal_radius,
+    )
+
+# Cleanup
+del install_import_hook, RUNTIME_TYPECHECKER
diff --git a/tests/smoke/dynamics/test_package.py b/tests/smoke/dynamics/test_package.py
index 231292e9..f0783c73 100644
--- a/tests/smoke/dynamics/test_package.py
+++ b/tests/smoke/dynamics/test_package.py
@@ -9,6 +9,7 @@ def test_all() -> None:
         # modules
         "fields",
         "integrate",
+        "cluster",
         "mockstream",
         "plot",
         # core