One of the most important unanswered questions in evolutionary biology is why sexual reproduction is so common. In theory, sex is so costly that individuals that reproduce sexually should be at a major disadvantage relative to asexually reproducing competitors. This leads to the expectation that sex should lose in competition with asexual reproduction and thus be rare in nature, which is in striking contrast to its widespread occurrence. It is this profound disconnect between theory and reality that has motivated decades of research into potential advantages of sexual reproduction. While much progress has come from these efforts, why sex predominates is still unclear. It is likely that part of the answer is linked to an advantage of sex under pressure from coevolving parasites. Other important processes may be more efficient clearance of deleterious mutations and other costs associated with asexuality that are not direct consequences of the lack of recombination. The fundamental difference between sexual and asexual reproduction is that sex produces offspring that have both unique combinations of alleles and different combinations of alleles than their parents and their siblings. In theory, sex should be rare because it breaks up allele combinations that have proven to work well together and because sexual females waste resources on son production. The predominance of sex in nature indicates that production of genetically distinct offspring must also generate profound benefits. Comparing the geographic distribution of sexual and asexual populations can provide insights into the advantages and disadvantages of sex. The most direct and powerful way to study sex is to compare coexisting sexual and asexual females that are phylogenetically close relatives and ecologically very similar with the exception of their reproductive mode. Likely direct benefits of sex include evasion of biological enemies and clearance of harmful mutations. The transition to asexuality may lower the fitness of asexual phenotypes and thus the cost of sex, in which case the maintenance of sex is easier to explain.