Sexual reproduction isan ancient feature of life on earth,and the familiar X and Y chromo-somes in humans and other modelspecies have led to the impressionthat sex determination mecha-nisms are old and conserved. Infact, males and females are deter-mined by diverse mechanisms thatevolve rapidly in many taxa. Yetthis diversity in primary sex-deter-miningsignalsiscoupledwithconserved molecular pathways thattrigger male or female develop-ment. Conflicting selection on dif-ferent parts of the genome and onthe two sexes may drive many ofthese transitions, but few systemswith rapid turnover of sex determi-nation mechanisms have been rig-orously studied. Here we surveyour current understanding of howand why sex determination evolvesin animals and plants and identifyimportant gaps in our knowledgethat present exciting research op-portunities to characterize the evo-lutionary forces and molecularpathways underlying the evolutionof sex determination.