In this chapter, we explore the evolutionary puzzle of how cooperation and other social behaviours can arise under the pressures of natural selection. The chapter begins by reviewing the concept of fitness, distinguishing between its direct and indirect components, and defining beneficial and costly behaviours in the context of social interactions. Drawing on various examples, we explore different mechanisms of cooperation, including direct, indirect, and generalised reciprocity, and discuss the associated costs and benefits for the individuals involved. Finally, we define relatedness between individuals, discuss the theory of kin selection, and explain Hamilton’s rule and its importance for understanding how altruistic behaviour evolves, particularly among related individuals.