diff --git a/2021-python/solutions/day_03.py b/2021-python/solutions/day_03.py
new file mode 100644
index 0000000..586446f
--- /dev/null
+++ b/2021-python/solutions/day_03.py
@@ -0,0 +1,43 @@
+from collections import Counter
+from solutions import BaseSolution
+
+
+class Solution(BaseSolution):
+    input_file = "03.txt"
+
+    def __str__(self):
+        return "Day 3: Binary Diagnostic"
+
+    def parse_input(self, data):
+        return data.split()
+
+    def solve(self, puzzle_input):
+        e, g = self._eg(puzzle_input)
+        return int("".join(g), 2) * int("".join(e), 2)
+
+    def solve_again(self, puzzle_input):
+        og = self._ogco2(puzzle_input, "1")
+        co2 = self._ogco2(puzzle_input, "0")
+        return int("".join(og), 2) * int("".join(co2), 2)
+
+    def _eg(self, puzzle_input):
+        e = []
+        g = []
+        for r in zip(*puzzle_input):
+            x, y = Counter(r).most_common()
+            e.append(x[0])
+            g.append(y[0])
+        return e, g
+
+    def _ogco2(self, values, default, i=0):
+        eg = [Counter(r).most_common() for r in zip(*values)]
+        k = default if eg[i][0][1] == eg[i][1][1] else eg[i][abs(int(default) - 1)][0]
+        rem = [v for v in values if v[i] == k]
+        if len(rem) == 1:
+            return rem
+        return self._ogco2(rem, default, i + 1)
+
+
+if __name__ == "__main__":
+    solution = Solution()
+    solution.show_results()
diff --git a/2021-python/tests/test_day_03.py b/2021-python/tests/test_day_03.py
new file mode 100644
index 0000000..ab4341d
--- /dev/null
+++ b/2021-python/tests/test_day_03.py
@@ -0,0 +1,41 @@
+import unittest
+
+from solutions.day_03 import Solution
+
+
+class Day03TestCase(unittest.TestCase):
+    def setUp(self):
+        self.solution = Solution()
+        self.puzzle_input = self.solution.parse_input(
+            """
+        00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010
+        """
+        )
+
+    def test_parse_puzzle_input(self):
+        data = """
+        00100
+11110
+        """
+        assert self.solution.parse_input(data) == ["00100", "11110"]
+
+    def test_solve_first_part(self):
+        assert self.solution.solve(self.puzzle_input) == 198
+
+    def test_solve_second_part(self):
+        assert self.solution.solve_again(self.puzzle_input) == 230
+
+
+if __name__ == "__main__":
+    unittest.main()