This chapter provides a general overview of the main mechanisms through which sexual selection operates, as reflected in the mating behaviour of crustaceans. Competition occurs when two or more individuals are dependent on the same resource and this resource is limited. While this term is broadly used in ecology, behavioural ecologists have been specifically interested in the underlying behaviour involved in competitive interactions and the consequences for the individual animal. Resource defence in crustaceans usually involves some type of refuge such as burrows, crevices or cavities. A well-studied example is the defence of breeding burrows in fiddler crabs. Resource defence also occurs in species of a solitary nature such as clawed lobsters and stomatopods. American lobsters, Homarus americanus, live in solitary shelters where they take refuge from predators and strong water currents. Female mate choice in crustaceans takes various forms and females use a variety of criteria, often in combination.