This chapter provides an overview of current theories about why sexual reproduction is so widespread in nature, exploring both the costs and evolutionary benefits of sex. It explains how sex can generate novel genetic combinations-bringing together beneficial mutations carried by different individuals, but also breaking up beneficial combinations. I also discuss how the evolutionary advantage of sex may depend on factors such as the size of the adapting population and the adaptive challenges it faces. The chapter concludes with a discussion of how experimental evolution with microorganisms such as yeast provides exciting new opportunities to partially test hypotheses about the evolution of sex.