Sex differences in spatial ability, especially mental rotation, navigation and object-location memory are described in this essay. Biological differences in brain morphology, hormones and genes between men and women are presented as explanations for the sex differences. Another level of explanations offered are evolutionary, hence the most influential evolutionary psychological theories are summarized and evaluated. These theories are Gaulin’s and Fitzgerald’s male range theory, Silverman’s and Eals’s hunter-gatherer theory, and Ecuyer-Dab’s and Robert’s twofold selection theory. The hunter-gatherer theory at present seems to be of the most importance, though the twofold selection theory may in the future challenge it. Regardless, united biological and evolutionary explanations would create the best comprehensive theory.