The radiation of light from matter rotating in the vicinitiy of compact objects (neutron stars or black holes) is expected to be affected from significant frecuency shifts which are two-fold in origin: (1) the velocity of rotation, in the inner parts of accretion disks, is of the order of the velocity of light, thereby producing Doppler shifts of pure kinematic origin. (2) the emitted radiation is affected from an additional redshift produced by the gravitational field of the central object. In this region, with a typical size of a few Schwarzschild radii, the local emitted radiation is shifted towards the blue or red while the action of the gravitational field always induce a shift towards lower frequencies. Due to the importance of these effects for an eventual confrontation with the observations, the authors shall briefly examine the essential aspects both in the kinematic situation as well as in the kinematic plus gravitational (general relativistic) case.