The problem of interaction among many agents and its impact on decision making is presented. There is an introduction to the basic concepts of game theory and to the different types of games depending on the nature of interaction (static, dynamic or repeated) as well as on the type of information agents have (perfect or imperfect). The main equilibrium concepts are studied (Nash equilibrium, sub game perfect equilibrium and Bayesian equilibrium) with economic applications: noncompetitive markets, political competition, bilateral bargaining, auctions, voting systems and the problem of cooperation in repeated games.