There are essentially four types of breaking waves, as described by Galvin (1968) and many others. The first two types are the plunging breaker, in which the wave crest curls forward and plunges into the slope of the wave at some distance away from the crest; and the spilling breaker, in which the broken region tends to develop more gently from an instability at the crest and often forms a quasi-steady whitecap on the forward face of wave. The third type, surging, sometimes develops as waves are incident upon a sloping beach. A fourth type of breaking, collapsing, is considered by many to be special limiting case of the plunging breaker. Several advances toward an understanding of wave breaking have been made in recent years. These include the experimental characterization of the instability mechanisms which lead to wave breaking in deep water, proposed mathematical models for these instability mechanisms, and numerical simulations of wave overturning and incipient breaking. These topics are discussed here. Little is known quantitatively about the processes of air entrainment in breaking waves. However, a more complete knowledge of the air entrainment process will be required for the further development of plausible models for radar scattering from breaking waves on the ocean surface. A forecase model for the breaking of deep water waves has been proposed and a breaking critierion for use with the model has been formulated. Recent laboratory experiments to study the onset of wave breaking in deep water provide one means for quantifying this criterion in terms of a breaking coefficient. An alternate means for determining this coefficient has been proposed on the basis of stream function wave theory. These topics also are discussed here.