Parasites are thought to play a role(s) in the evolution of host traits. Evidence for their roles comes from studies examining parasite-mediated selection. Odonates are model insects and water mites and gregarine protozoans model parasites, for such investigations. These parasites can exert fitness costs on their dragonfly hosts such as reduced survivorship and affect signals and possibly also mating success of males, but more experimental work is needed. Resistance against ectoparasitic mites is governed by the parasite-host association being considered and by temperature following host emergence, but not by host gender. Other work suggests host gender and age influence immunological responses to bacterial and artificial challenges. Recent evidence with dragonflies suggests that the likelihood of demonstrating parasite-mediated selection might depend on whether or not the parasite species being considered is a generalist parasite. Ideas also are emerging on how prey species should deal with co-occurring threats from multiple enemies such as predators and parasites. Here again, dragonflies and their parasites should provide tests of developing theory.