The present state of research into and understanding of shear waves is appraised. In this paper these motions, which result from an instability of an alongshore, wave-driven current, are described, their theoretical explanation is recounted, and the history of their discovery is related. The various investigations into their genesis, which attempted to develop an understanding of when and why they occur, are summarized, and more recent research into their finite amplitude development is discussed, focusing on the understanding gained. Attention is also given to observations of shear waves made in the field and attempts to observe them in the laboratory. Finally, work that still needs to be done is described. ¿ 2000 American Geophysical Union