Binary systems containing a black hole or a neutron star offer the best possibility to test prediction of General Relativity in the strong field regime. The plasma stripped from the non-degenerate companion star reaches the space time in the immediate vicinity of the compact object and releases strong X-ray emission. The spectral and variability properties of this emission contain the signatures of predicted effects such as the presence of an innermost stable orbit and black hole spin. I discuss the current observational status with particular emphasis onto sub-second time variability, which constitutes the most direct measurement of the properties of the plasma accreting onto a collapsed star.