The visualization of sound fields has recently attracted the attention of scientists and engineers of a wide range of disciplines. Consequently, a brief introduction to the physical principles and practical methods involved in light and sound holography will serve to point out the similarities to and diversities from each other. The two proven methods for forming acoustical holograms, the "free surface" method and the "scanning" method will be discussed in detail. Other less perfected methods will also be included in the discussion. Most of the latter methods require rather high acoustical intensities, and this will lead to a number of non-linear effects, which result finally in phase distortion. Various applications of acoustic holography are enumerated.