All animals face the decision of when, where, and how much to invest in reproduction. This investment can vary as a function of clutch interval, clutch size, and egg size. Consequently, variation in reproductive investment can refer to variation in any of these characters, which can be observed to different degree among species, among populations within species, and among individuals within populations. The processes that shape this variation are of great interest to evolutionary ecologists. This is not only because reproduction is one of the most fundamental aspects of organismal biology, but also because variation in reproductive investment enables tests of adaptive and non-adaptive models of phenotypic evolution and provides insights into the interplay between demographic, ecological, and evolutionary change.