Magnetars are neutron stars with ultra-strong magnetic fields, that emit regular gamma-ray flares powered by field decay. Quasi-periodic oscillations (QPOs) detected after highly energetic giant flares have now confirmed that these events are associated with large-scale starquakes, each QPO representing a particular mode of vibration of the neutron star. I will summarize the observed properties of the QPOs, and outline what observational tests are possible with data from current satellites.
