Daphnia and its parasites have become recognized as a model system for studying the epidemiological, evolutionary and genetic interactions between hosts and parasites. The key advantages of the Daphnia-parasite system are the propagation of the host as iso-female lines, that is clonal, but at the same time the possibility to cross lines. Furthermore, Daphnia have diverse parasites, including bacteria, fungi, microsporidia and helminths, which can be kept in culture with the hosts. For two parasites of Daphnia magna, coevolution has been demonstrated phenotypically. Coevolution in D. magna and the bacterium Pasteuria ramosa is consistent with model predictions of coevolution by negative frequency dependent selection, the Red Queen hypothesis. The genetic mechanisms have not yet been elucidated.