© 2008 Elsevier B.V. All rights reserved. Kin selection describes a process by which a gene can spread through a population through social effects on reproductive success. Kin selection provides an explanation for how a gene for altruism can spread even if it lowers the fitness of the carrier. The process depends on the fact that beneficiaries of an altruistic act have a higher probability of sharing a gene relative to the population as a whole. The basic theory of kin selection is outlined, examples of evidence supporting the theory given, and general applications of the theory are described.