While the ability to reproduce is shared across all domains of life, the particular way in which animals reproduce varies among species, from the familiar to what might strike us as truly bizarre. To take just one of many examples: Soon after it is born, a male anglerfish finds a female anglerfish, bites her skin, and fuses with her. His body atrophies, leaving the male reproductive organs physically attached to the female and allowing her eggs to be fertilized when she is ready. To begin to make sense of this and other reproductive strategies, we start by considering how different forms of reproduction evolved, what organisms use them, and their adaptive significance.