This chapter introduces the modeling framework for analyzing strategic interactions in large populations of decision-making agents. The framework consists of three fundamental elements: the population game, which specifies the strategic environment; the payoff mechanism, which determines how agents perceive rewards; and the revision process, which governs how agents update their choices over time. The considered setting emphasizes decentralized decision-making, where aggregate behaviors emerge through repeated interactions rather than through centralized control. Fundamental properties of the framework, such as large populations, finite roles, and negligible individual influence, are highlighted to define its scope of applicability. Two central questions are posed: the existence of equilibrium states and the convergence of populations toward such equilibria. Addressing these questions provides theoretical insights into collective behaviors and informs the design of mechanisms for coordination and control in large-scale multi-agent systems. The chapter concludes by outlining the book’s structure, presenting motivating examples, and introducing the generalized Nash equilibrium as the solution concept of interest.