The coupling of optical and mechanical degrees of freedom is the underlying principle of many techniques to measure mechanical displacement, from macroscale gravitational wave detectors to microscale cantilevers used in scanning probe microscopy. Recent experiments have reached a regime where the back- action of photons caused by radiation pressure can influence the optomechanical dynamics, giving rise to a host of long- anticipated phenomena. Here we review these developments and discuss the opportunities for innovative technology as well as for fundamental science.