One of the most prominent hypotheses to explain the ubiquity of sex and recombination is based on host-parasite interactions. Under the name of the Red Queen hypothesis (RQH), it has had theoretical and empirical support since its conception, but recent theoretical work has shown that the circumstances under which the RQH works remain unclear. Here we review the current status of the theory of the RQH. We argue that recent theoretical work calls for new experimental data and an increased theoretical effort to reveal the driving force of the RQH.