diff --git a/pyfixest/did/saturated_twfe.py b/pyfixest/did/saturated_twfe.py
new file mode 100644
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+++ b/pyfixest/did/saturated_twfe.py
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+from typing import Optional
+
+import matplotlib.pyplot as plt
+import numpy as np
+import pandas as pd
+
+from pyfixest.estimation.estimation import feols
+
+from .did2s import DID
+
+
+class SaturatedEventStudy(DID):
+    """
+    Saturated event study with cohort-specific effect curves.
+
+    Attributes
+    ----------
+    data : pd.DataFrame
+        Dataframe containing the data.
+    yname : str
+        Name of the outcome variable.
+    idname : str
+        Name of the unit identifier variable.
+    tname : str
+        Name of the time variable.
+    gname : str
+        Name of the treatment variable.
+    cluster : str
+        The name of the cluster variable.
+    xfml : str
+        The formula for the fixed effects.
+    att : bool
+        Whether to use the average treatment effect.
+
+    """
+
+    def __init__(
+        self,
+        data: pd.DataFrame,
+        yname: str,
+        idname: str,
+        tname: str,
+        gname: str,
+        cluster: Optional[str] = None,
+        xfml: Optional[str] = None,
+        att: Optional[bool] = True,
+    ):
+        super().__init__(
+            data=data,
+            yname=yname,
+            idname=idname,
+            tname=tname,
+            gname=gname,
+            cluster=cluster,
+            xfml=xfml,
+            att=att,
+        )
+        self._estimator = "Saturated Event Study"
+
+        # create a treatment variable
+        self._data["ATT"] = (self._data[self._tname] >= self._data[self._gname]) * (
+            self._data[self._gname] > 0
+        )
+
+    def estimate(self):
+        """
+        Estimate the model.
+
+        Returns
+        -------
+        pd.DataFrame
+            A DataFrame containing the estimates.
+        """
+        self.mod = _saturated_event_study(
+            self._data,
+            outcome=self._yname,
+            treatment="ATT",
+            time_id=self._tname,
+            unit_id=self._idname,
+        )
+
+        return self.mod
+
+    # !TODO - implement the rest of the methods
+    def vcov(self):
+        """
+        Get the covariance matrix.
+
+        Returns
+        -------
+        pd.DataFrame
+            A DataFrame containing the covariance matrix.
+        """
+        pass
+
+    def iplot(self):
+        """Plot DID estimates."""
+        pass
+
+    def tidy(self):
+        """Tidy result dataframe."""
+        return self.mod.tidy()
+
+    def summary(self):
+        """Get summary table."""
+        return self.mod.summary()
+
+
+######################################################################
+
+
+def _saturated_event_study(
+    df: pd.DataFrame,
+    outcome: str = "outcome",
+    treatment: str = "treated",
+    time_id: str = "time",
+    unit_id: str = "unit",
+    ax: plt.Axes = None,
+):
+    ######################################################################
+    # this chunk creates gname internally here - assume that data already contains it?
+    df = df.merge(
+        df.assign(first_treated_period=df[time_id] * df[treatment])
+        .groupby(unit_id)["first_treated_period"]
+        .apply(lambda x: x[x > 0].min()),
+        on=unit_id,
+    )
+    df["rel_time"] = df[time_id] - df["first_treated_period"]
+    df["first_treated_period"] = (
+        df["first_treated_period"].replace(np.nan, 0).astype("int")
+    )
+    df["rel_time"] = df["rel_time"].replace(np.nan, np.inf)
+    cohort_dummies = pd.get_dummies(
+        df.first_treated_period, drop_first=True, prefix="cohort_dummy"
+    )
+    df_int = pd.concat([df, cohort_dummies], axis=1)
+    ######################################################################
+    # formula
+    ff = f"""
+                {outcome} ~
+                {'+'.join([f"i(rel_time, {x}, ref = -1.0)" for x in df_int.filter(like = "cohort_dummy", axis = 1).columns])}
+                | {unit_id} + {time_id}
+                """
+    m = feols(ff, df_int, vcov={"CRV1": unit_id})
+    # !TODO move this into a separate function / integrate logic into iplot method that overrides feols's plot
+    if ax:
+        # plot
+        res = m.tidy()
+        # create a dict with cohort specific effect curves
+        res_dict = {}
+        for c in cohort_dummies.columns:
+            res_cohort = res.filter(like=c, axis=0)
+            event_time = (
+                res_cohort.index.str.extract(r"\[T\.(-?\d+\.\d+)\]")
+                .astype(float)
+                .values.flatten()
+            )
+            res_dict[c] = {"est": res_cohort, "time": event_time}
+
+        cmp = plt.get_cmap("Set1")
+        for i, (k, v) in enumerate(res_dict.items()):
+            ax.plot(v["time"], v["est"]["Estimate"], marker=".", label=k, color=cmp(i))
+            ax.fill_between(
+                v["time"], v["est"]["2.5%"], v["est"]["97.5%"], alpha=0.2, color=cmp(i)
+            )
+        ax.axvline(-1, color="black", linestyle="--")
+        ax.axhline(0, color="black", linestyle=":")
+    return m
+
+
+def test_treatment_heterogeneity(
+    df: pd.DataFrame,
+    outcome: str = "Y_it",
+    treatment: str = "W_it",
+    unit_id: str = "unit_id",
+    time_id: str = "time_id",
+    retmod: bool = False,
+):
+    """
+    Test for treatment heterogeneity in the event study design.
+
+    For details, see https://github.com/apoorvalal/TestingInEventStudies
+
+    Parameters
+    ----------
+    df : pd.DataFrame
+        Dataframe containing the data.
+    outcome : str
+        Name of the outcome variable.
+    treatment : str
+        Name of the treatment variable.
+    unit_id : str
+        Name of the unit identifier variable.
+    time_id : str
+        Name of the time variable.
+    retmod : bool
+        Whether to return the model object.
+
+    Returns
+    -------
+    float
+        The p-value of the test.
+    """
+    # Get treatment timing info
+    df = df.merge(
+        df.assign(first_treated_period=df[time_id] * df[treatment])
+        .groupby(unit_id)["first_treated_period"]
+        .apply(lambda x: x[x > 0].min()),
+        on=unit_id,
+    )
+    df["rel_time"] = df[time_id] - df["first_treated_period"]
+    df["first_treated_period"] = (
+        df["first_treated_period"].replace(np.nan, 0).astype("int")
+    )
+    df["rel_time"] = df["rel_time"].replace(np.nan, np.inf)
+    # Create dummies but drop TWO cohorts - one serves as base for pooled effects
+    cohort_dummies = pd.get_dummies(
+        df.first_treated_period, drop_first=True, prefix="cohort_dummy"
+    ).iloc[
+        :, 1:
+    ]  # drop an additional cohort - drops interactions for never treated and baseline
+
+    df_int = pd.concat([df, cohort_dummies], axis=1)
+
+    # Modified formula with base effects + cohort-specific deviations
+    ff = f"""
+    {outcome} ~
+    i(rel_time, ref=-1.0) +
+    {'+'.join([f"i(rel_time, {x}, ref = -1.0)" for x in df_int.filter(like = "cohort_dummy", axis = 1).columns])}
+    | {unit_id} + {time_id}
+    """
+
+    model = feols(ff, df_int, vcov={"CRV1": unit_id})
+    P = model.coef().shape[0]
+
+    if retmod:
+        return model
+    mmres = model.tidy().reset_index()
+    mmres[["time", "cohort"]] = mmres.Coefficient.str.split(":", expand=True)
+    mmres["time"] = mmres.time.str.extract(r"\[T\.(-?\d+\.\d+)\]").astype(float)
+    mmres["cohort"] = mmres.cohort.str.extract(r"(\d+)")
+    # indices of coefficients that are deviations from common event study coefs
+    event_study_coefs = mmres.loc[~(mmres.cohort.isna()) & (mmres.time > 0)].index
+    # Method 2 (K x P) - more efficient
+    K = len(event_study_coefs)
+    R2 = np.zeros((K, P))
+    for i, idx in enumerate(event_study_coefs):
+        R2[i, idx] = 1
+
+    test_result = model.wald_test(R=R2, distribution="chi2")
+    return test_result["pvalue"]
+
+
+def _test_dynamics(
+    df,
+    outcome="Y",
+    treatment="W",
+    time_id="time",
+    unit_id="unit",
+    vcv={"CRV1": "unit"},
+):
+    # Fit models
+    df = df.merge(
+        df.assign(first_treated_period=df[time_id] * df[treatment])
+        .groupby(unit_id)["first_treated_period"]
+        .apply(lambda x: x[x > 0].min()),
+        on=unit_id,
+    )
+    df["rel_time"] = df[time_id] - df["first_treated_period"]
+    df["rel_time"] = df["rel_time"].replace(np.nan, np.inf)
+    restricted = feols(f"{outcome} ~ i({treatment}) | {unit_id} + {time_id}", df)
+    unrestricted = feols(
+        f"{outcome} ~ i(rel_time, ref=0) | {unit_id} + {time_id}", df, vcov=vcv
+    )
+    # Get the restricted estimate
+    restricted_effect = restricted.coef().iloc[0]
+    # Create R matrix - each row tests one event study coefficient
+    # against restricted estimate
+    n_evstudy_coefs = unrestricted.coef().shape[0]
+    R = np.eye(n_evstudy_coefs)
+    # q vector is the restricted estimate repeated
+    q = np.repeat(restricted_effect, n_evstudy_coefs)
+    # Conduct Wald test
+    pv = unrestricted.wald_test(R=R, q=q, distribution="chi2")["pvalue"]
+    return pv