diff --git a/policyengine_core/reforms/scenario.py b/policyengine_core/reforms/scenario.py
new file mode 100644
index 00000000..1e5cfac6
--- /dev/null
+++ b/policyengine_core/reforms/scenario.py
@@ -0,0 +1,20 @@
+from pydantic import BaseModel, Field
+from typing import TYPE_CHECKING, Callable
+
+if TYPE_CHECKING:
+    from policyengine_core.simulations import Simulation
+
+
+class Scenario(BaseModel):
+    parameter_changes: (
+        dict[str, dict[str | int, float | int | bool]] | None
+    ) = None
+    """A dictionary mapping parameter names to their time-period-specific changes."""
+    modifier_function: Callable[["Simulation"], None] | None = None
+    """A function that modifies the simulation in some way, e.g., by applying a tax policy change."""
+
+    def __init__(self):
+        # Validate parameter changes
+        pass
+        # Validate modifier function?
+        pass
diff --git a/policyengine_core/simulations/simulation.py b/policyengine_core/simulations/simulation.py
index a0b1ec85..160e7752 100644
--- a/policyengine_core/simulations/simulation.py
+++ b/policyengine_core/simulations/simulation.py
@@ -6,7 +6,7 @@
 from numpy.typing import ArrayLike
 import logging
 from pathlib import Path
-
+from policyengine_core.reforms.scenario import Scenario
 from policyengine_core import commons, periods
 from policyengine_core.data.dataset import Dataset
 from policyengine_core.entities.entity import Entity
@@ -91,6 +91,7 @@ def __init__(
         trace: bool = False,
         default_input_period: str = None,
         default_calculation_period: str = None,
+        scenario: Scenario = None,
     ):
         self.default_input_period = (
             default_input_period or self.default_input_period
@@ -227,6 +228,12 @@ def __init__(
 
         self.parent_branch = None
 
+        if scenario is not None:
+            # Apply parameter changes
+            pass
+            if scenario.modifier_function is not None:
+                scenario.modifier_function(self)
+
     def apply_reform(self, reform: Union[tuple, Reform]):
         if isinstance(reform, tuple):
             for subreform in reform: