In view of its fundamental and pervasive influences and impacts on organism physiology and ecology, body size is recognized as a key component of evolutionary fitness and serves as the cornerstone of a seminal contribution in freshwater zooplankton ecology-the Size Efficiency Hypothesis (SEH) of Brooks & Dodson (Science 150:28-35, 1965). While the roles and implications of body size in predation and competition-central tenets of the SEH-have been widely considered and reviewed, no broader integrated synthesis exists of the collective array of body size determinants and their implications in the ecology in crustacean zooplankton-a numerically and functionally dominant group of aquatic organisms. Focusing on planktonic Cladocera and Copepoda in inland waters, in particular, we provide a wide-ranging overview of the direct and/or indirect effects of environmental conditions, consumable resources and biotic interactions that independently and/or collectively influence the phenotypic expression of body size (particularly as length), both within and between species. Some indirect ultimate evolutionary consequences of body size are considered, and we identify some controversies and unresolved issues related to this biologically crucial trait. While by no means exhaustive, our overview reveals a complex nexus of extrinsic proximate abiotic and biotic factors and interactions that influence body size, the phenotypic expression of which in natural systems commonly reflects contrasting outcomes related to conflicting direct and/or indirect selective pressures. In general, however, body size (both interand intra specifically) declines with rising temperature and increases with rising food supply (depending on its quality), although both temperature and food supply exert contrary influences on particular taxa (or life history stages) under certain environmental circumstances. Predation undoubtedly has an overriding influence on body size selection. Depending on its mechanistic basis (visual, tactile or both in tandem), it selectively favours either small or large body size, both within (adults vs. juveniles) and between prey species, which are accordingly often 'size-trapped' between contrasting selective pressures, with consequent indirect effects. The bioenergetics of fundamental physiological processes undoubtedly set constraints on body size and serve as the primary determinant. However, within such