This chapter generalizes the concept of strategic stability by introducing the Nash equilibrium—a solution concept that applies even when players have no dominant or dominated strategies. It teaches how to use best response analysis to find equilibria in a wide range of games, such as assurance, chicken, and the tragedy of the commons, with applications from espionage to resource sharing and electoral alliances. The chapter extends equilibrium reasoning to continuous strategy spaces, relevant for business competition in prices, quantities, and product design. It closes with a look at evolutionary game theory, which offers a powerful alternative to classical assumptions of rationality by focusing on long-term dynamics in large populations.