Recent theoretical and experimental knowledge concerning oscillatory boundary layers is reviewed in this paper, and an assessment made of the present state of our understanding of this particular form of unsteady flow. Although agreement is found between theory and experiment for the case of laminar oscillatory flow over a smooth flat bed, our theoretical knowledge concerning other types of oscillatory flow in both laminar and turbulent conditions is shown to be inadequate. Some attention is given to the problem of defining the transition between a truly laminar condition and a fully developed turbulent one for unsteady flow over beds of different roughness and form. Attention is also focused on the increase in boundary shear stress that arises from frequency effects.