Ian Ball continues MIT's 14.12 Economic Applications of Game Theory with a session on multi-stage games. He introduces the one-shot deviation principle, the tool that lets economists check whether a strategy profile is a subgame perfect equilibrium by testing single deviations at each decision point rather than every possible alternative strategy. The lecture builds from the formal statement of the principle toward its application in bargaining models, showing how repeated offers and counteroffers can be analyzed stage by stage. Ball works through the underlying logic on the board, connecting the technique back to backward induction and extensive-form games covered earlier in the course. Runtime runs about 78 minutes, consistent with a full class session rather than a highlight reel, and the material assumes familiarity with basic game-theoretic notation from prior lectures in the sequence.