This paper explores the fundamental problem of inferring the outcome of a noncooperative game from the rationality and information of the players. The solution concept introduced is called "rationalizability," which is not always equivalent to Nash equilibrium. The paper argues that some Nash equilibria are intuitively unreasonable, and not all reasonable strategy profiles are Nash equilibria. It discusses the limitations of Nash equilibrium as a criterion for rationality and proposes a broader definition of equilibrium that accounts for imperfections. The paper introduces two solution concepts: rationalizability and cautious rationalizability, which differ in their treatment of "unreasonable" behavior. Rationalizability is based on logical deduction, while cautious rationalizability adds an assumption of prudence to eliminate certain types of imperfections. The paper provides a detailed development of these concepts in both normal form and extensive form games, showing how they can be used to analyze strategic behavior and predict outcomes.This paper explores the fundamental problem of inferring the outcome of a noncooperative game from the rationality and information of the players. The solution concept introduced is called "rationalizability," which is not always equivalent to Nash equilibrium. The paper argues that some Nash equilibria are intuitively unreasonable, and not all reasonable strategy profiles are Nash equilibria. It discusses the limitations of Nash equilibrium as a criterion for rationality and proposes a broader definition of equilibrium that accounts for imperfections. The paper introduces two solution concepts: rationalizability and cautious rationalizability, which differ in their treatment of "unreasonable" behavior. Rationalizability is based on logical deduction, while cautious rationalizability adds an assumption of prudence to eliminate certain types of imperfections. The paper provides a detailed development of these concepts in both normal form and extensive form games, showing how they can be used to analyze strategic behavior and predict outcomes.