Terrestrial vertebrates are constantly losing water to their environment, including water lost through the skin as cutaneous water loss. The outermost layer of the skin, the stratum corneum, contains multiple layers of lipids, and the composition of these lipids determines an animal's rate of cutaneous water loss. In this talk, we'll explore how lipids in the stratum corneum interact to prevent cutaneous water loss in a variety of animals while also discussing how some of these lipids may play additional roles in the skin that have shaped major evolutionary transitions.