This chapter introduces game theory as a framework for understanding strategic interaction in biological, social, and economic systems. Beginning with the foundations of zero-sum games and the minimax theorem, it explains how rational behavior can be analyzed when outcomes depend on the actions of others. The chapter then turns to John Nash’s generalization to non-zero-sum games, introducing Nash equilibrium as a stable outcome in which no player can benefit by unilaterally changing strategy. Classic examples such as the prisoner’s dilemma illustrate the tension between individual rationality and collective welfare. Moving beyond one-shot games, the chapter explores repeated interactions and shows how cooperation can emerge over time, even among self-interested agents. Strategies such as tit-for-tat demonstrate how reciprocity, punishment, and forgiveness can stabilize cooperative behavior. The discussion then extends to evolutionary game theory, where strategies spread or disappear based on reproductive success rather than conscious choice. Concepts such as evolutionarily stable strategies explain how population-level patterns arise in biology and behavior. Together, these ideas reveal game theory as a unifying logic underlying evolution, cooperation, conflict, and decision-making across living systems.