A theory is presented which indicates that the quasi-biennial oscillation of the zonal wind in the tropical stratosphere is a result of the interaction of long-period, vertically propagating gravity waves with the zonal wind. We discuss the theoretical basis and observational evidence for the existence of long-period gravity waves near the equator, and the mechanism of their interaction with the zonal wind, and present some simple numerical results which show how the absorption of the momentum of these waves by the mean flow leads to a downward propagating zonal wind profile. It is shown that the interaction of these gravity waves with the observed semiannual zonal wind oscillation above 40 km will produce a downward propagating quasi-biennial oscillation. We present the results of several numerical experiments with a model of the tropical stratosphere which includes the gravity wave interaction mechanism. The quasi-biennial oscillation is simulated quite successfully. Finally, we discuss possible observational checks for our model, and some of its implications for tropical dynamics.