In part I, an introductory chapter outlines the scope of reproductive phenomena in nature and explains why evolutionary biologists pay special attention to creatures that incubate their progeny. The remaining four chapters in part I then provide additional facts (and some mythologies) about the diverse expressions of pregnancy-like phenomena in a wide variety of vertebrate and nonvertebrate organisms (including humans) that brood embryos. This information about incubation serves as an empirical backdrop for part II, which addresses pregnancy’s many expressions from a comparative evolutionary perspective.