The focus of this chapter is suicide, with emphasis on two evolutionary explanations. One views suicide as a by-product of states, traits, or predispositions that may have been adaptive in ancestral environments. The other views suicide as an evolved response that is conditioned on rare but well-specified environmental circumstances. The chapter considers this second approach—an inclusive fitness model of self-destructive motivation—in some detail. This model identifies circumstances under which self-preservation might be expected to break down—namely, the coupling of low individual reproductive potential with a sense of being a burden to close kin (burdensomeness). Although the model was designed to show how self-destructive phenomena might have evolved in ancestral environments, there is growing evidence to suggest that it is also capable of predicting instances of suicidal thinking and behavior in the present. We review that evidence and include a description of our own program of research that has tested and extended the model.