Numerous synchronized time series from video cameras, pressure sensors, current meters, and hot film anemometers on natural beaches show that boundary layer development under the crest of near-breaking waves can be idealized as a process composed of three distinct regimes here referred to as streaking, roiling, and pluming. The roiling and pluming regimes fail to develop under the trough. As a consequence, there is a pronounced asymmetry in instantaneous sand transport and boundary layer phenomena between the wave crest and trough. However, laboratory waves with field scale periods and wave heights over thin sand beds do not exhibit this crest-trough boundary layer asymmetry, indicating that a critical element of similitude is absent in laboratory experiments. We suggest that wave induced boundary ventilation is responsible. ¿ American Geophysical Union 1992