Reproduction-passing your genes to the next generation-is the name of the game in evolution. If your genes are not passed on, your reproductive fitness is zero. Sexual reproduction is a very old phenomenon; it evolved in the sea, long before the vertebrates. Almost all vertebrates reproduce sexually and sex is one of the most powerful of all drives. (In browsing through this book, you probably turned to this chapter before any other, right?) Sexual reproduction ensures high levels of genetic diversity in the offspring and thus provides the raw material upon which natural selection can act. Sexual reproduction on land has desiccation constraints not found with aquatic sex, and terrestrial vertebrates have evolved various strategies to protect their eggs and developing embryos. In this chapter we examine how vertebrates reproduce sexually, and in the next chapter we will see how the fertilized egg develops into an adult.