We propose a simple empirical model to describe the Rayleigh-Taylor (RT) turbulent mixing of immiscible, miscible and stratified fluids. For immiscible fluids, the rate of momentum loss is an invariant of the flow, whereas the energy dissipation rate increases linearly with time. Turbulent diffusion, accounted for through temperature fluctuations, does not terminate mixing but slows it down significantly. A stratified density distribution can stabilize RT mixing. (c) 2005 American Institute of Physics.