Surveys previous studies on the turbulent region formed at the boundary between two parallel fluid streams of different velocity and discusses observations made on the large scale vortex structure. Presents results for longitudinal velocity fluctuations at two different downstream stations, frequency spectra, and velocity cross correlations. Estuaries that the energy associated with the quasi two-dimensional vortices is at least 50% of the (fluctuating) turbulent kinetic energy. Examines the geometry of the large vortices and presents isometric plots of velocity field and topological maps. Discusses how vortex interactions centrol the turbulent momentum transport, and how large changes in mixing layer properties are produced by weak, but spatially coherent forcing. (C.J.U.)