The development of concepts of individual death and species extinction were examined in two studies. Sixty-eight, 4to 9-year-old children and their parents participated in Study 1. Although preschoolers had some understanding of the concept of death, the ability to explain death and extinction improved over the school-age years. However, all age groups were reluctant to accept that extinction may be inevitable for all species, despite the ease with which they accepted death as inevitable for all living beings. This resistance was especially strong in the case of human extinction. Study 2 extended these findings with three groups of adults possessing different degrees of biological expertise. Unlike medical students and lay-adults, evolutionary biologists endorsed the inevitability of extinction and generalized this belief to humans. The results suggest that for the lay-public, death and extinction concepts elicit existential concerns and that they are embedded in potentially conflicting intuitive theories: metaphysics and biology. © 2004 Elsevier Inc. All rights reserved.