Quasi periodic oscillations have been detected in the X-ray emission of many BHXBs and are thought to originate in the innermost regions of the accretion flow around the black hole. Even though their origin and nature is still debated, the study of QPOs provides a way to explore the inner accretion flow around black holes and neutron stars. Few theoretical models have been proposed to explain the origin of QPOs, but only one has been proved to be promising so far, having shown good agreement with observations. I will briefly describe timing in X-rays and the study of QPOs. Then I will introduce the relativistic precession model to explain the origin of LFQPOs and I will show the results of the test of this model to the observations, describing its implications and showing how ultimately it allows to obtain almost direct measures of Mass and spin of the central black hole.