▪ Abstract  Recently, there has been exciting progress in our understanding of the behavioral and evolutionary ecology of immature parasitoids. Developing parasitoids face a diversity of ecological constraints, and parasitism success involves decisions and responses made by immature parasitoids to find a host and solve conflicts with five potential antagonists: host, mother, siblings, competitors, and natural enemies. In this review we synthesize and interpret results from studies on (a) the convergent evolution of host selection behavior of first-instar larvae and females in hymenopteran, dipteran, and coleopteran families; (b) the competitive interactions between larval parasitoids and the evolution of gregariousness; (c) the susceptibility of parasitized hosts to predation; and (d) the ability of parasitoids to manipulate the behavior of the host. We discuss how ecological interactions between juvenile parasitoids and their hosts, competitors, and natural enemies influence the evolution of parasitoid life-history strategies, and why the integration of functional aspects of the ecology of immature parasitoids provides a reliable framework for effective host-parasitoid population models and formulation of biological control solutions.