The circulation of the stratosphere, and its influence on the trace constituent distribution, is an important component of the climate system, which must be included in simulations of global climate change. However, the ability to simulate a dominant stratospheric phenomenon, the Quasi-Biennial Oscillation (QBO) in equatorial zonal wind, is an outstanding challenge in climate modeling. Although confined to the tropics, the QBO affects the circulation and the interannual variability of the entire stratosphere, parts of the mesosphere and possibly also of the troposphere. Here we show that the QBO is successfully simulated in a general circulation model (GCM) of the newest generation. Key factors are a sufficient spatial resolution, a realistic simulation of tropical convection, and the consideration of the effects of gravity waves. From this simulation it is inferred that a broad spectrum of atmospheric waves is necessary to generate the QBO in the model.