Spiders, dung-feeding scarabs, social, and prey-storing insects provide predictable and concentrated sources of food for a variety of thief flies (kleptoparasites) and their larvae. Whenever waiting in the vicinity of the "host" for an opportunity to exploit its resources is more energy efficient and less dangerous than foraging among hosts, a number of intimate relationships between the fly and host may evolve. In extreme cases, flies may become long-term phoretic associates that travel with hosts even while the latter is in flight. The behaviors and ecologies of kleptoparasitic Diptera are reviewed with special attention paid to the adaptations of Sphaeroceridae phoretic upon Scarabaeidae. The mating systems of kleptoparasitic flies are influenced by the type of resource that is stolen; flies associated with predators are mostly female, while those found on scarabs are of both sexes. These differences are discussed in terms of mate location, sperm competition, and mate choice.