Gravitational waves are deformations of space-time. The beating of the local clock of an inertial observer with the time given by photons travelling between two points is sensitive to these waves. Actual instruments consists in Michelson interferometers with Fabry-Perot cavities in the arms. The thermal noise of the mirror suspensions and of the mirror bulks limit the spectral density of the resolution in the 10 Hz - 200 Hz range; the detected shot noise is the resolution limit in the 200 Hz - 10 kHz range. We give the status of actual interferometers. Advanced detectors, in operation in the next decade, open new technical challenges.