Fast time variability is an important characteristic of black hole X-ray binaries and a key ingredient in understanding the physical processes in these systems. Black hole X-ray binaries show a variety of X-ray spectral/variability states, representing different accretion regimes and it has been recently shown that the overall strength of the rapid variability is a good tracer of these states. Fast aperiodic variability is generally studied through the inspection of power density spectra. Most of the power spectral components are broad and can take the form of a wide power distribution over several decades of frequency or of a more localised peak (quasi-periodic oscillations, QPOs). It is now clear that QPOs are a common characteristic of accreting systems: they have been observed in accreting stellar mass black holes and neutron stars hosted in X-ray binaries, in cataclysmic variable, in the so-called ultra luminous X-ray sources and even in active galactic nuclei. 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. Various theoretical models have been proposed to explain the origin of QPOs in black hole binaries, only a few proved to be promising so far, having shown good agreement with observations. I will describe how timing is used in X-rays to probe accretion onto compact objects and how QPOs are usually studied. I will briefly review some of the proposed models and in particular the so-called Relativisti Precession Model, based on a few basic predictions of GR. I will show how this model can be used to estimate the masses and most-importantly the spins of accreting black holes, which so far have been measured only through indirect ways.