The processes which lead to vertical mixing in stratified fluids operate in a multiparameter space, much of which has not been explored theoretically or in laboratory experiments. This review focuses on the transitional phenomena which precede, and may eventually lead to, the broadband spectra associated with turbulent flow. Conditions of static instability are known to be produced in wave-wave, wave-current, and wave-boundary interactions, but little is known of the subsequent stages of transition. Whilst there has been considerable progress in describing transition in Rayleigh-Benard and Taylor-Couette flows, that in Kelvin-Helmholtz instability is relatively unexplored, and in consequence, our knowledge of energy exchange and of density and momentum fluxes is extremely sparse. Attention is drawn here to the flow disturbance caused by sidewalls in laboratory experiments, and some recently discovered examples of transitional phenomena in Kelvin-Helmholtz instability are described. More information is available about fluxes in double-diffusive convection, but here too the transitional processes are as yet poorly known. Applying this information to the natural environment (and emphasis is given to the oceans) poses the fundamental difficulty of estimating the probability of conditions favoring the development of any particular kind of instability. Knowledge of transition in conditions in which the background field varies in space and time is deficient. ¿American Geophysical Union 1987