Unsteadiness of flow at M2-S2 tidal frequencies is demonstrated to be a dominant factor forming eddies in natural, shallow-water and continental shelf environments. Numerical hydrodynamic modelling and field measurements show that during the later stages of the half tidal cycle in a low-friction environment, currents in the lee of an obstruction accelerate with supporting sea gradients opposite to the adjacent, inertial free stream, which is still decelerating. These opposed currents appear as a phase eddy and ideally can endure for 3 hours in M2-S2 reversing currents. Case studies are cited, and numerical hydrodynamic modelling is applied to examine the formation of these circulation patterns. ¿ American Geophysical Union 1987