Wasps (Vespidae) exhibit a range of social complexity, from solitary living to eusocial colonies, and thus are exemplary for studies of the evolutionary origin and maintenance of social behavior in animals. Integral to the definition of eusociality is the presence of reproductive castes, group members that differ qualitatively in their ability to reproduce in a social setting. Behavioral and morphological evidence suggests that caste determination, the developmental process by which differences in fecundity are established, occurs to a large extent before adult emergence (pre-imaginally) in many species of Vespidae, in both basal and advanced taxa within the clade (Vespinae+Polistinae), which includes most eusocial species. Pre-imaginal determination has been documented in many taxa (e.g. independent-founding Polistinae) where it was not thought to occur. Correlative and experimental studies indicate that differences in nutrition during larval development are often the basis of pre-imaginal caste determination. Pre-imaginal caste determination has important implications for the roles of subfertility and manipulation by nest mates in the evolution of eusocial behavior.