Host-parasitoid interactions are abundant in nature and offer great scope for the study of coevolution. A particularly fertile area is the interaction between internal feeding parasitoids and their hosts. Hosts have evolved a variety of means of combating parasitoids, in particular cellular encapsulation, while parasitoids have evolved a wide range of countermeasures. Studies of the evolution of host resistance and parasitoid virulence are reviewed, with an emphasis on work involving Drosophila and its parasitoids. Genetic variation in both traits has been demonstrated using isofemale line and artificial selection techniques. Recent studies have investigated the fitness costs of maintaining the ability to resist parasitoids, the comparative fitness of flies that have successfully defended themselves against parasitoids, and the degree to which resistance and virulence act against one or more species of host or parasitoid. A number of studies have examined geographical patterns, and sought to look for local adaptation; or have compared the traits across a range of species. Finally, the physiological and genetic basis of change in resistance and virulence is being investigated. While concentrating on Drosophila, the limited amount of work on different systems is reviewed, and other possible areas of coevolution in host-parasitoid interactions are briefly discussed.