Theory predicts that sexual reproduction is difficult to maintain if asexuality is an option, yet sex is very common. To understand why, it is important to pay attention to repeatably occurring conditions that favour transitions to, or persistence of, asexuality. Geographic parthenogenesis is a term that has been applied to describe a large variety of patterns where sexual and related asexual forms differ in their geographic distribution. Often asexuality is stated to occur in a habitat that is in some sense marginal, but the interpretation differs across studies: parthenogens might predominate near the margin of the sexuals’ distribution but might also extend far beyond the sexual range, they may be disproportionately found in newly colonizable areas (e.g., areas previously glaciated), or in habitats where abiotic selection pressures are relatively stronger than biotic ones (e.g. cold, dry). Here we review the various patterns proposed in the literature, the hypotheses put forward to explain them, and the assumptions they rely on. Surprisingly few mathematical models consider geographic parthenogenesis as their focal question, but all models for the evolution of sex could be evaluated in this framework if the (often ecological) causal factors vary predictably with geography. We also recommend broadening the taxa studied beyond the traditional favourites.