Most crustaceans pass through a complex life cycle, commonly comprising a benthic juvenile-adult and a pelagic larval phase. As a consequence of radical transitions in life style, different ontogenetic stages of the same species may differ greatly in ecology, behaviour, functional morphology, anatomy, and physiology. The interdisciplinary field of larval biology analyzes developmental changes in all those traits, as well as persisting effects of larval condition on postmetamorphic fitness. Larval biology is thus not only an intrinsic subject of life-history studies, but also it contributes essential information to various other disciplines within the broad area of crustacean research. Comparative studies of larval morphology, for example, aid the identification of phylogenetic relationships among higher taxa. In extreme cases (e.g. parasitic cirripedes), even the affiliation to the Crustacea could only be shown with larval characters. Patterns of larval behaviour, dispersal and recruitment determine population connectivity and, in consequence, genetic diversity and formation of metapopulations. Knowledge of these aspects is fundamental for the understanding of biogeographic distributions, population and community stability, and ecosystem structure. In analyses of limnic and terrestrial invasions, developmental traits in relation to ecophysiology reflect past evolutionary life-history transitions, indicate future invasive potentials, or suggest incipient speciations. Similarly, the spreading of introduced species in new environments may be explained or predicted with patterns of larval development and tolerance of particular environmental conditions. In economically relevant species, larval biology is critical for the management of sustainable fisheries and the development of aquaculture techniques.