The biological life history (LH) theory has been increasingly utilized in psychology, especially in developmental psychology. However, there has not been a comprehensive text on the topic thatalso addresses applications in psychology. This Element fills this void. Organized into five sections, it initially delineates and explains the species-general concepts and principles forming LH theory, emphasizing that, although derived from observations between species, they can be used to explain individual differences within human populations. Grounded in the assumption of phenotypic plasticity, subsequent LH research conducted in psychology covers a wide range of cognitive and social behavioral domains. This body of LH research is discussed next. The Element concludes by presenting four broad recommendations, which, comprising one-quarter of the total content, provide specific directions for future LH research in psychology.