Our Milky Way Galaxy is surrounded by dozens of smaller galaxies. By proximity, the Milky Way’s satellite galaxy population includes the faintest and lowest mass systems in the known Universe. These “ultra-faint” galaxies are several million times fainter than the Milky Way itself, providing a rare window into how the first galaxies formed, and they may help us test ideas about dark matter—the invisible material thought to make up most of the matter in the universe. In this talk, I’ll trace the discovery of ultra‑faint galaxies around the Milky Way over the past decade, and preview new surveys that may reveal even fainter companions in the next few years. Finally, I’ll show how these surprising objects are reshaping a basic question: what, exactly, counts as a galaxy?