One of the most prominent characteristics of acoustic signals transmitted from one point to another in the ocean is the fluctuation of the signal, in both space and time. This is true for both fixed and moving source and receiver. Postulated mechanisms and controlling parameters for various modes of propagation and types of environment are discussed. The environment includes the nature of reflectors and scatterers and their influence on the fluctuation. Examples of appropriate measures of fluctuation are offered. Experiments for testing mechanisms and obtaining descriptive data are discussed. Some problems of prediction are outlined.