Cooperation is a fundamental aspect of human behavior but is evolutionarily rare and cannot be explained easily. Humans cooperate in a way that is difficult to explain using standard models from evolutionary biology. The methods used in the study of the evolution of cooperation allow for the examination of complex systems through simplification. Four broad classes of models have received an enormous amount of attention in the study of the evolution of cooperation: inclusive fitness, reciprocal altruism, indirect reciprocity, and group/multilevel selection. Future directions in the study of the evolution of cooperation include considering multiple simple models in the same framework—including the study of linked games and the evolution of cooperation on multiplex networks.