diff --git a/src/stats/histogram.rs b/src/stats/histogram.rs
new file mode 100644
index 0000000..91162e9
--- /dev/null
+++ b/src/stats/histogram.rs
@@ -0,0 +1,81 @@
+use std::marker::PhantomData;
+
+use super::Categorical;
+
+/// [Histogram] is a data structure for tracking categorical observations.
+/// In essence, its a map from Category to count.
+/// A [Histogram] has a fixed number of bins, each representing on of `N`
+/// categories. Each time an member of the category is observed, it is
+/// "added to the bin", incrementing the counter.
+pub struct Histogram<const N: usize, C>
+where
+    C: Categorical<N>,
+{
+    bins: Box<[usize; N]>,
+    phantom: PhantomData<C>,
+}
+
+impl<const N: usize, C> Histogram<N, C>
+where
+    C: Categorical<N>,
+{
+    /// Create a new histogram with zeroes across the board.
+    pub fn new() -> Self {
+        let bins = Box::new([0; N]);
+        Self {
+            bins,
+            phantom: PhantomData,
+        }
+    }
+
+    /// Increment the observed count for the given category by 1.
+    pub fn increment(&mut self, categorical: &C) {
+        self.increment_by(categorical, 1);
+    }
+
+    /// Increment the observed count for the given category by `count`.
+    pub fn increment_by(&mut self, categorical: &C, count: usize) {
+        let index = categorical.category();
+        self.bins[index] += count;
+    }
+
+    /// Return the count for the given category.
+    pub fn get_count(&self, categorical: &C) -> usize {
+        let index = categorical.category();
+        self.bins[index]
+    }
+
+    /// Reset all bins to zero.
+    pub fn clear(&mut self) {
+        self.bins = Box::new([0; N]);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::stats::Categorical;
+
+    use super::Histogram;
+
+    impl Categorical<2> for bool {
+        fn category(&self) -> usize {
+            match self {
+                true => 1,
+                false => 0,
+            }
+        }
+    }
+
+    #[test]
+    fn test_increment() {
+        let mut hist = Histogram::new();
+        // start at 0.
+        assert_eq!(hist.get_count(&true), 0);
+        assert_eq!(hist.get_count(&true), 0);
+        hist.increment(&true);
+        hist.increment(&true);
+        hist.increment(&false);
+        assert_eq!(hist.get_count(&true), 2);
+        assert_eq!(hist.get_count(&false), 1);
+    }
+}
diff --git a/src/stats/mod.rs b/src/stats/mod.rs
index bd2a6b6..182b598 100644
--- a/src/stats/mod.rs
+++ b/src/stats/mod.rs
@@ -106,6 +106,8 @@ mod categorical;
 mod chi;
 /// `group` defines the two groups.
 mod group;
+/// A data structure for tracking categorical data.
+mod histogram;
 /// An observation represents a group and the observed category.
 mod observation;
 /// Different kinds of contingency tables.