Host-parasitoid interactions are abundant in nature and offer great sc
ope for the study of coevolution. A particularly fertile area is the i
nteraction between internal feeding parasitoids and their hosts. Hosts
have evolved a variety of means of combating parasitoids, in particul
ar cellular encapsulation, while parasitoids have evolved a wide range
of countermeasures. Studies of the evolution of host resistance and p
arasitoid Virulence are reviewed, with an emphasis on work involving D
rosophila and its parasitoids. Genetic variation in both traits has be
en demonstrated using isofemale line and artificial selection techniqu
es. Recent studies have investigated the fitness costs of maintaining
the ability to resist parasitoids, the comparative fitness of flies th
at have successfully defended themselves against parasitoids, and the
degree to which resistance and virulence act against one or more speci
es of host or parasitoid. A number of studies have examined geographic
al 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 investiga
ted. 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.