Over the past two decades, observational studies using aircraft and satellite data to supplement conventional surface and upper air data have revealed much about the structure of tropical cyclones. During this same period, numerical weather prediction models have been developed to study the dynamics and energetics of these intense, destructive storms. This article reviews the structure of mature tropical cyclones and discusses the physical processes that are important in their genesis and maintenance. Some examples of computer simulation models are presented. Finally, the operational prediction of actual tropical storms by statistical and dynamical methods is summarized.