Kelvin-Helmholtz (KH) instability, characterized by the distinctive finite-amplitude billows it generates, is an important mechanism in the development of turbulence in the stratified interior of the ocean. In particular, it is often assumed that the onset of turbulence in internal waves begins in this way. Clear recognition of the importance of KH instability to ocean mixing arises from recent observations of the phenomenon in a broad range of oceanic environments. KH instability is a critical link in the chain of events that leads from internal waves to mixing. After 150 years of research, identifying the prevalence of KH instability in the ocean and defining useful parameterizations that quantify its contribution to ocean mixing in numerical models remain first-order problems.