Game Theory analyzes strategic interactions where multiple decision-makers' choices affect each other's outcomes. Relevant to OR in: competitive facility location, supply chain coordination, pricing strategies, and auction design. Key concepts: (1) Nash Equilibrium - no player can improve by unilaterally changing strategy; (2) Zero-sum games - one player's gain equals another's loss, solved via linear programming; (3) Cooperative games - players form coalitions, using solution concepts like Shapley value. Applications: pricing competition, resource allocation in shared infrastructure, and mechanism design for efficient outcomes. Game-theoretic models inform: competitive strategy, contract design, and understanding market equilibria. Modern topics include: algorithmic game theory and optimization in multi-agent systems.