Maximiliano Isi, of Columbia University and the Flatiron Institute, surveys how gravitational wave observations are being used to test general relativity itself rather than just catalog mergers. Speaking at the MPG-IAS-NTU Center Kick-off Conference in Wolfensohn Hall, he walks through three testing grounds: using gravitational waves as standard sirens for cosmological distance measurements, extracting the ringdown signal left after two black holes merge to check whether the resulting object behaves exactly as Einstein's equations predict, and the gravitational memory effect, a permanent spacetime distortion left behind after a wave passes. Isi explains what current LIGO and Virgo data can and cannot yet resolve, and where next-generation detectors would close the gap. The talk assumes some familiarity with general relativity but keeps the three strands distinct and concrete, giving a working picture of how black hole ringdowns are becoming a direct probe of gravity in the strong-field regime.