MIT economist Ian Ball teaches this session of 14.12 Economic Applications of Game Theory, covering the Folk Theorem for infinitely repeated games. He works through why, when players are patient enough, almost any feasible and individually rational payoff can be supported as a Nash or subgame perfect equilibrium. The lecture builds the logic through punishment strategies and the role of discounting, showing how the threat of future retaliation sustains cooperation that would collapse in a single shot version of the same game. Ball develops the formal conditions step by step on the board, connecting the abstract result back to real strategic settings like collusion and repeated bargaining. Runs 78 minutes as part of MIT's Fall 2025 game theory course, released through MIT OpenCourseWare.