The concept of sexual segregation was 1st formally proposed by Charles Darwin. Among mammals, ruminants have been the focus of most research on this phenomenon. Sexual segregation has been defined traditionally as the differential use of space (and often habitat and forage) by sexes outside the mating season, but other hypotheses related to activity patterns of sexes recently have been forwarded. These new hypotheses, however, cannot explain the spatial separation of sexes or their differential use of habitats and forages. Sexual segregation should be considered in a niche framework wherein overlap on 1 niche axis is accompanied by avoidance on another, including space, diet, and habitat. Moreover, sexual segregation is scale sensitive, which limits the usefulness of a comparative approach in investigating differences among species or populations. Failure to discriminate between the potential evolutionary causes of sexual dimorphism in ruminants has led to confusion over whether polygyny or intersexual competition has led to sexual segregation. Neither exploitive nor interference competition, wherein males are competitively excluded by females, is a likely cause of sexual segregation. I suggest that the gastrocentric model, which invokes allometric and life-history differences between sexes, or risk of predation are the only hypotheses necessary to explain sexual segregation. Additional research, however, is necessary to verify some aspects of those hypotheses. The management and conservation of ruminants requires consideration that sexes behave as if they were different species, which holds consequences for estimating populations, manipulating harvest and habitats, and the potential spread of diseases and parasites and might have genetic consequences for some populations. This review attempts to resolve long-standing problems related to studies of sexual segregation, but much research remains to be accomplished, including more manipulative experiments.