Ian Ball closes MIT's 14.12 Economic Applications of Game Theory with the concept of common knowledge, the idea that all players not only know a fact but know that everyone else knows it, and know that everyone knows they know it, in an infinite regress. He works through why this recursive structure matters for strategic reasoning, distinguishing common knowledge from mere mutual knowledge and showing how the gap between the two changes what rational players can conclude from observing each other's actions. Expect blackboard derivations and formal examples rather than slides, consistent with the course's style, building on the semester's earlier game-theoretic tools to show how epistemic assumptions shape equilibrium outcomes. As the final lecture of the term, it serves as a capstone on how information structure, not just payoffs, drives strategic behavior in economic models.