This chapter discusses the interactions generating and dissipating internal waves, the primary mechanism mixing the stratified ocean. Generation is primarily by wind stress at the surface, bottom stress at the seafloor, and tidal flows over irregular bottoms. Energy is transferred from large to small scales by resonant interactions, primary triads, that lead to ultimate breaking and mixing. Testing of expressions for predicting dissipation rates resulting from wave–wave interactions is examined in detail. The mechanisms of breaking affect the efficiency of mixing and therefore the buoyancy fluxes it produces. Shear instabilities and advective overturning are the two primary mechanisms, but the form of instability remains open to question. Finally, the saturated range separating linear internal waves from turbulence is examined.