Dengue virus is a mosquito-borne flavivirus found throughout tropical and subtropical regions. Four distinct serotypes are in circulation. Time series data from Bangkok show that incidence of all four serotypes oscillates with a ten year epidemic cycle. However, three serotypes oscillate in phase, the fourth almost perfectly out of phase. A stochastic two-strain SIR model forced with an annually varying transmission rate will be introduced. This generates multi-year epidemic cycles and indicates that antiphase solutions only occur for a limited range of interserotypic cross- immunity. The implications of this result will be discussed and phylogenetic data will be presented that suggest the antiphase epidemic pattern may play an important role in the evolution of dengue virus. The phase patterns found in the stochastic model also occur as resonant solutions of a deterministic form of the model, although the epidemic period is shorter. Work in progress to understand the mechanisms behind these deterministic phase patterns will be discussed.