Field and laboratory observations were carried out on the reproductive behaviour of Cyclograpsus lavauxi and Helice crassa, and the results were compared with other Grapsidae, with emphasis on New Zealand species. Mating in all species typically occurs during the intermoult and often coincides with the time of oviposition. Females of several species have been reported to mate multiple times, often in the few days prior to oviposition, leading to sperm competition within the female spermatheca. Females were found to be sexually receptive only in a short period before oviposition (e.g., C. lavauxi, Hemigrapsus crenulatus, H. sexdentatus), although some species were also receptive for about two weeks after oviposition (e.g., H. crassa). The exact duration of female receptivity is unknown for most grapsid species. Female grapsid crabs exhibit a wide range of gonopore structures which either restrict female receptivity to certain times or allow them to mate at any time. In species with restricted female receptivity (e.g., C. lavauxi, H. crassa, H. crenulatus, H. sexdentatus), the operational sex ratio is typically highly male-biased. Male-male competition was found to be intense in the four New Zealand grapsids, with frequent male-male interactions during which larger males were typically more successful in fights over females, resulting in a greater number of matings. Postcopulatory guarding, which is assumed to reduce the risk of sperm competition and to ensure paternity, has been observed in a few grapsid species (H. sexdentatus and H. crenulatus) but not in others (e.g., C. lavauxi and H. crassa). Overall, grapsid crabs employ a variety of reproductive strategies including direct competition between males for females, with postcopulatory guarding; males securing resources as sites for mating; and males having only brief interceptions with receptive females. These differences in reproductive behaviour are discussed in the context of sexual selection and the ecological and environmental differences of the habitats that grapsid crabs occupy. Mating strategies of grapsid species can be very different even if they occupy the same habitat and the females have similar duration of sexual receptivity.