An astrophysical black hole is fully defined by just two parameters : its mass and spin. With the advent of modern X-ray satellites such as XMM-Newton and Chandra came a leap in our understanding of the fundamental physics of these powerful objects. We are now at a stage where information on the mass and spin of black holes is available for nearly a dozen X-ray binaries (XRBs), and we are in the process of obtaining the spin for a similar number of supermassive black holes. X-ray observations of stellar-mass black holes provide three independent means from which we can estimate the black hole spin : i) thermal emission emanating from the accretion disk ; ii) relativistic distortion to emission lines ; and iii) quasi-periodic variations in the light originating in the system. In this talk, I will summarize the current status of the field after reviewing and discussing the validity and physical assumptions of these methods. The implication of the current spin distribution for stellar mass black holes will also be discussed.