diff --git a/.github/workflows/check_jupyterbook.yml b/.github/workflows/check_jupyterbook.yml
index d2174d5..49f4e0d 100644
--- a/.github/workflows/check_jupyterbook.yml
+++ b/.github/workflows/check_jupyterbook.yml
@@ -15,7 +15,7 @@ jobs:
         with:
           activate-environment: microdf
           environment-file: environment.yml
-          python-version: 3.8
+          python-version: 3.9
           auto-activate-base: false
 
       - name: Build  # Build Jupyter Book
diff --git a/.github/workflows/deploy_jupyterbook.yml b/.github/workflows/deploy_jupyterbook.yml
index 677b49a..f78dcc5 100644
--- a/.github/workflows/deploy_jupyterbook.yml
+++ b/.github/workflows/deploy_jupyterbook.yml
@@ -18,7 +18,7 @@ jobs:
         with:
           activate-environment: microdf
           environment-file: environment.yml
-          python-version: 3.8
+          python-version: 3.9
           auto-activate-base: false
 
       - name: Build
diff --git a/.github/workflows/linting.yml b/.github/workflows/linting.yml
index c887c4e..133ddf6 100644
--- a/.github/workflows/linting.yml
+++ b/.github/workflows/linting.yml
@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.8]
+        python-version: [3.9]
 
     steps:
       - name: Checkout
diff --git a/microdf/inequality.py b/microdf/inequality.py
index 765a451..3259f17 100644
--- a/microdf/inequality.py
+++ b/microdf/inequality.py
@@ -3,7 +3,7 @@ import numpy as np
 import microdf as mdf
 
 
-def gini(df, col, w=None, negatives=None):
+def gini(df, col, w=None, negatives=None, groupby=None):
     """Calculates Gini index.
 
     :param df: DataFrame.
@@ -16,96 +16,117 @@ def gini(df, col, w=None, negatives=None):
         when this minimum is negative. That is, it adds the absolute
         minimum value.
         Defaults to None, which leaves negative values as they are.
+    :param groupby: Column, or list of columns, to group by.
+    
     :returns: A float, the Gini index.
 
     """
-    # Requires float numpy arrays (not pandas Series or lists) to work.
-    x = np.array(df[col]).astype("float")
-    if negatives == "zero":
-        x[x < 0] = 0
-    if negatives == "shift" and np.amin(x) < 0:
-        x -= np.amin(x)
-    if w is not None:
-        w = np.array(df[w]).astype("float")
-        sorted_indices = np.argsort(x)
-        sorted_x = x[sorted_indices]
-        sorted_w = w[sorted_indices]
-        cumw = np.cumsum(sorted_w)
-        cumxw = np.cumsum(sorted_x * sorted_w)
-        return np.sum(cumxw[1:] * cumw[:-1] - cumxw[:-1] * cumw[1:]) / (
-            cumxw[-1] * cumw[-1]
-        )
-    else:
-        sorted_x = np.sort(x)
-        n = len(x)
-        cumxw = np.cumsum(sorted_x)
-        # The above formula, with all weights equal to 1 simplifies to:
-        return (n + 1 - 2 * np.sum(cumxw) / cumxw[-1]) / n
-
-
-def top_x_pct_share(df, col, top_x_pct, w=None):
+
+    def _gini(df, col, w=None, negatives=None):
+        # Requires float numpy arrays (not pandas Series or lists) to work.
+        x = np.array(df[col]).astype("float")
+        if negatives == "zero":
+            x[x < 0] = 0
+        if negatives == "shift" and np.amin(x) < 0:
+            x -= np.amin(x)
+        if w is not None:
+            w = np.array(df[w]).astype("float")
+            sorted_indices = np.argsort(x)
+            sorted_x = x[sorted_indices]
+            sorted_w = w[sorted_indices]
+            cumw = np.cumsum(sorted_w)
+            cumxw = np.cumsum(sorted_x * sorted_w)
+            return np.sum(cumxw[1:] * cumw[:-1] - cumxw[:-1] * cumw[1:]) / (
+                cumxw[-1] * cumw[-1]
+            )
+        else:
+            sorted_x = np.sort(x)
+            n = len(x)
+            cumxw = np.cumsum(sorted_x)
+            # The above formula, with all weights equal to 1 simplifies to:
+            return (n + 1 - 2 * np.sum(cumxw) / cumxw[-1]) / n
+
+    if groupby is None:
+        return _gini(df, col, w, negatives)
+    return df.groupby(groupby).apply(lambda x: _gini(x, col, w, negatives))
+
+
+def top_x_pct_share(df, col, top_x_pct, w=None, groupby=None):
     """Calculates top x% share.
 
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param top_x_pct: Decimal between 0 and 1 of the top %, e.g. 0.1, 0.001.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the top x%.
 
     """
-    threshold = mdf.weighted_quantile(df, col, w, 1 - top_x_pct)
-    top_x_pct_sum = mdf.weighted_sum(df[df[col] >= threshold], col, w)
-    total_sum = mdf.weighted_sum(df, col, w)
-    return top_x_pct_sum / total_sum
+
+    def _top_x_pct_share(df, col, top_x_pct, w=None):
+        threshold = mdf.weighted_quantile(df, col, w, 1 - top_x_pct)
+        top_x_pct_sum = mdf.weighted_sum(df[df[col] >= threshold], col, w)
+        total_sum = mdf.weighted_sum(df, col, w)
+        return top_x_pct_sum / total_sum
+
+    if groupby is None:
+        return _top_x_pct_share(df, col, top_x_pct, w)
+    return df.groupby(groupby).apply(
+        lambda x: _top_x_pct_share(x, col, top_x_pct, w)
+    )
 
 
-def bottom_x_pct_share(df, col, bottom_x_pct, w=None):
+def bottom_x_pct_share(df, col, bottom_x_pct, w=None, groupby=None):
     """Calculates bottom x% share.
 
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param bottom_x_pct: Decimal between 0 and 1 of the top %, e.g. 0.1, 0.001.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the bottom x%.
 
     """
-    return 1 - top_x_pct_share(df, col, 1 - bottom_x_pct, w, top=False)
+    return 1 - top_x_pct_share(df, col, 1 - bottom_x_pct, w, groupby)
 
 
-def bottom_50_pct_share(df, col, w=None):
+def bottom_50_pct_share(df, col, w=None, groupby=None):
     """Calculates bottom 50% share.
 
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the bottom 50%.
 
     """
-    return bottom_x_pct_share(df, col, 0.5, w)
+    return bottom_x_pct_share(df, col, 0.5, w, groupby)
 
 
-def top_50_pct_share(df, col, w=None):
+def top_50_pct_share(df, col, w=None, groupby=None):
     """Calculates top 50% share.
 
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the top 50%.
 
     """
-    return top_x_pct_share(df, col, 0.5, w)
+    return top_x_pct_share(df, col, 0.5, w, groupby)
 
 
-def top_10_pct_share(df, col, w=None):
+def top_10_pct_share(df, col, w=None, groupby=None):
     """Calculates top 10% share.
 
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the top 10%.
 
     """
-    return top_x_pct_share(df, col, 0.1, w)
+    return top_x_pct_share(df, col, 0.1, w, groupby)
 
 
 def top_1_pct_share(df, col, w=None):
@@ -114,32 +135,37 @@ def top_1_pct_share(df, col, w=None):
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the top 1%.
 
     """
     return top_x_pct_share(df, col, 0.01, w)
 
 
-def top_0_1_pct_share(df, col, w=None):
+def top_0_1_pct_share(df, col, w=None, groupby=None):
     """Calculates top 0.1% share.
 
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the top 0.1%.
 
     """
-    return top_x_pct_share(df, col, 0.001, w)
+    return top_x_pct_share(df, col, 0.001, w, groupby)
 
 
-def t10_b50(df, col, w=None):
+def t10_b50(df, col, w=None, groupby=None):
     """Calculates ratio between the top 10% and bottom 50% shares.
 
     :param df: DataFrame.
     :param col: Name of column in df representing value.
     :param w: Column representing weight in df.
+    :param groupby: Column, or list of columns, to group by.
     :returns: The share of w-weighted val held by the top 10% divided by
         the share of w-weighted val held by the bottom 50%.
 
     """
-    return top_10_pct_share(df, col, w) / bottom_50_pct_share(df, col, w)
+    t10 = top_10_pct_share(df, col, w, groupby)
+    b50 = bottom_50_pct_share(df, col, w, groupby)
+    return t10 / b50

diff --git a/.github/workflows/build_and_test.yml b/.github/workflows/build_and_test.yml
index 04146a8..daeab97 100644
--- a/.github/workflows/build_and_test.yml
+++ b/.github/workflows/build_and_test.yml
@@ -1,4 +1,4 @@
-name: Build and test [Python 3.6, 3.7, 3.8]
+name: Build and test [Python 3.7, 3.8, 3.9]
 
 on: [push, pull_request]
 
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.7, 3.8, 3.9]
 
     steps:
       - name: Checkout
diff --git a/microdf/tests/test_weighted.py b/microdf/tests/test_weighted.py
index c481b4f..bc23251 100644
--- a/microdf/tests/test_weighted.py
+++ b/microdf/tests/test_weighted.py
@@ -49,3 +49,14 @@ def test_weighted_sum():
     # Test grouped.
     mdf.weighted_sum(dfg, "x", "w", "g")
     mdf.weighted_sum(dfg, ["x", "y"], "w", "g")
+
+
+def test_gini():
+    # Unweighted
+    mdf.gini(df, "x")
+    # Weighted
+    mdf.gini(df, "x", "w")
+    # Unweighted, grouped
+    mdf.gini(dfg, "x", groupby="g")
+    # Weighted, grouped
+    mdf.gini(dfg, "x", "w", groupby="g")

Test Python 3.9
Pending https://github.com/PSLmodels/Tax-Calculator/pull/2522
microdf/tests/test_weighted.py::test_gini
microdf/tests/test_weighted.py::test_weighted_quantile microdf/tests/test_weighted.py::test_weighted_median microdf/tests/test_weighted.py::test_weighted_mean microdf/tests/test_weighted.py::test_weighted_sum