diff --git a/source/add_binary/mod.rs b/source/add_binary/mod.rs
new file mode 100644
index 0000000..5d713c7
--- /dev/null
+++ b/source/add_binary/mod.rs
@@ -0,0 +1,68 @@
+pub fn add_binary(a: String, b: String) -> String {
+  // Pad the strings with leading zeroes so they're the same length.
+  let (a, b) = (
+    format!("{:0>1$}", a, b.len()),
+    format!("{:0>1$}", b, a.len()),
+  );
+
+  let mut carry = false;
+  let mut sum = String::new();
+
+  // Create reversed iterators for both binary strings.
+  let (mut a, mut b) = (a.chars().rev(), b.chars().rev());
+
+  while let Some(a) = a.next() {
+    // Safe to unwrap since we know both iterators will be the same length.
+    let b = b.next().unwrap();
+
+    match (a, b) {
+      ('1', '1') => {
+        // For 1 + 1, if we need to carry it will be 01 + 01 -> 10 + 01 -> 11.
+        // And otherwise it will be 01 + 01 -> 10, and carry next digit.
+        if carry {
+          sum.push('1');
+        } else {
+          sum.push('0');
+          carry = true;
+        }
+      }
+
+      // For 1 + 0 (and associatively 0 + 1), if we need to carry it's the same
+      // as 01 + 01 -> 10. And keep the carry set.
+      // And otherwise it will just be 01 + 00 -> 01.
+      ('1', '0') | ('0', '1') => {
+        if carry {
+          sum.push('0');
+        } else {
+          sum.push('1');
+        }
+      }
+
+      // For 0 + 0, if we need to carry it results in 1 and otherwise 0.
+      ('0', '0') => {
+        if carry {
+          sum.push('1');
+          carry = false;
+        } else {
+          sum.push('0');
+        }
+      }
+
+      _ => unreachable!(),
+    }
+  }
+
+  // And if the final digits required carrying, add the 1 here at the end.
+  if carry {
+    sum.push('1');
+  }
+
+  sum.chars().rev().collect()
+}
+
+#[test]
+fn test_add_binary() {
+  assert_eq!(add_binary("11".to_string(), "1".to_string()), "100");
+  assert_eq!(add_binary("1010".to_string(), "1011".to_string()), "10101");
+  assert_eq!(add_binary("1111".to_string(), "1111".to_string()), "11110");
+}
diff --git a/source/lib.rs b/source/lib.rs
index 8194462..51f671a 100644
--- a/source/lib.rs
+++ b/source/lib.rs
@@ -1,3 +1,4 @@
+pub mod add_binary;
 pub mod implement_strstr;
 pub mod length_of_last_word;
 pub mod longest_common_prefix;