This chapter explores the epistemic conditions required for a Nash equilibrium to be played in a game. It addresses the question of why Nash equilibrium is considered a useful concept, despite its apparent circularity. The authors argue that common knowledge of the game's structure, rationality, and strategies is not necessary for Nash equilibrium to hold. Instead, they focus on what players know or believe about each other's strategies, payoffs, and rationality. The paper introduces the concept of interactive belief systems as a framework for analyzing epistemic conditions. It shows that common knowledge of players' beliefs about each other's strategies is essential, but only when there are at least three players. The authors provide formal statements and proofs of their results, along with lemmas and counterexamples to illustrate the sharpness of their findings. The chapter also discusses the applicability of these results to both finite and infinite belief systems. It concludes with a discussion of the conceptual implications and related literature. The main ideas are summarized in sections "Description of the results" and "The main counterexamples," while more technical details are found in other sections. The paper aims to clarify the epistemic foundations of Nash equilibrium and demonstrate that weaker conditions than common knowledge are sufficient for equilibrium to be played.This chapter explores the epistemic conditions required for a Nash equilibrium to be played in a game. It addresses the question of why Nash equilibrium is considered a useful concept, despite its apparent circularity. The authors argue that common knowledge of the game's structure, rationality, and strategies is not necessary for Nash equilibrium to hold. Instead, they focus on what players know or believe about each other's strategies, payoffs, and rationality. The paper introduces the concept of interactive belief systems as a framework for analyzing epistemic conditions. It shows that common knowledge of players' beliefs about each other's strategies is essential, but only when there are at least three players. The authors provide formal statements and proofs of their results, along with lemmas and counterexamples to illustrate the sharpness of their findings. The chapter also discusses the applicability of these results to both finite and infinite belief systems. It concludes with a discussion of the conceptual implications and related literature. The main ideas are summarized in sections "Description of the results" and "The main counterexamples," while more technical details are found in other sections. The paper aims to clarify the epistemic foundations of Nash equilibrium and demonstrate that weaker conditions than common knowledge are sufficient for equilibrium to be played.