Repeating earthquakes are a nearly ubiquitous feature of the seismicity at active and erupting volcanoes worldwide. A seismic source is persistently reactivated, producing nearly identical waveforms that can be identified and clustered by cross-correlation methods. Changes in the timing, location, and waveforms of these earthquakes can be informative about ongoing and evolving processes within a volcano. In this talk, I'll highlight several different examples of the utility of repeating earthquakes to understand a "screaming" volcano in Alaska, changes in the shallow velocity structure of Mount St. Helens, and swarms of deep migrating earthquakes under Mammoth Mountain.