This chapter compares two sexual systems: hermaphroditism (each individual can produce gametes of either sex) and gonochorism (each individual produces gametes of only one of the two distinct sexes) in crustaceans. These two main sexual systems contain a variety of alternative modes of reproduction, which are of great interest from applied and theoretical perspectives. The chapter focuses on the description, prevalence, analysis, and interpretation of these sexual systems, centering on their evolutionary transitions. The ecological correlates of each reproductive system are also explored. In particular, the prevalence of “unusual” (non-gonochoristic) reproductive strategies has been identified under low population densities and in unpredictable/unstable environments, often linked to specific habitats or lifestyles (such as parasitism) and in colonizing species. Finally, population-level consequences of some sexual systems are considered, especially in terms of sex ratios. The chapter aims to provide a broad and extensive overview of the evolution, adaptation, ecological constraints, and implications of the various reproductive modes in this extraordinarily successful group of organisms.