The basic idea of Bill Hamilton’s kin selection theory is—as with so many brilliant ideas—quite simple. Bill Hamilton (1964a, b) argued that evolution is not solely determined by the reproduction of the individual (although, of course, individual reproduction is an important factor in evolution), but that the reproduction of close relatives can also indirectly contribute to the success of one’s own genes. For this reason, the theory was also called the theory of kin selection. This chapter is more concerned with evolutionary biology than with ecology. Here, I explain Bill Hamilton’s idea of kin selection and its outstanding significance for modern biology, especially for evolutionary biology and ecology. The theory states that in my behavior I am either selfish, or I favor others with whom I am closely related (the closer the kinship, the greater the cooperation). The flip side of favoring relatives is, of course, the discrimination against non-relatives.