Digital holographic microscopy (DHM) is a new optical method taking advantage of coherent properties of light propagation and it provides, without the use of any contrast agent, real-time three-dimensional images of transparent living cells, with an axial sensitivity of a few tens of nanometers. Specifically, both three-dimensional images of cellular surface morphometry and intracellular refractive index mapping are provided. They result from the hologram numerical reconstruction process, which permits a subwavelength calculation of the phase shift, produced on the transmitted wavefront, by the optically probed cells. Consequently, DHM offers unique possibilities for monitoring biological processes involving rapid and discrete morphological changes, including membrane nano-movements in specific cellular microdomains.