The ocean is a stably stratified medium, where the heavy fluid lies at the bottom with the lighter fluid on top. Mixing often occurs, however, that changes the stratification : in some places like the Denmark Strait or the Mediterranean outflow, heavier currents, as they flow down, entrain ambient fluid that results in density change of the current. We will discuss experiments that improve our understanding of some of the mechanisms involved in these phenomena. We begin by presenting results on hydrodynamic instability and mixing of gravity currents owing to Kelvin-Helmholtz and Holmboe instabilities where simultaneously measured velocity and density in a perpendicular plane allow a very complete quantitative characterization of the flow. We show how one can make close connection to idealized linear stability calculations through using the interface as the center around which to average. We use this approach to characterize the transition from strong mixing at low Richardson number via the Kelvin-Helmholtz instability to weak mixing at high Richardson number.