Parasitoids in the insect order Diptera include an estimated 16,000 species, or approximately 20% of the total number of species with this life-style. Parasitoids in this order are exceedingly diverse in both their habits and evolutionary origins, which makes them an underutilized but highly suitable group for quantitative studies of character convergence and adaptive radiation. This review focuses on several aspects of the bionomics of dipteran parasitoids that have received little comprehensive treatment, including processes associated with host location and attack, patterns of host use, and the evolutionary and ecological consequences of host-parasitoid interactions. Throughout the review we contrast the patterns found within the parasitic Diptera against those found in the better studied parasitic Hymenoptera. We conclude that more intensive study of dipteran parasitoids is required before we can understand the general conditions that favor the evolution of insect parasitoids and the truly magnifying themes of their behavior and ecology.