Perfect competition provides the model of a frictionless economy, in which price-setting economic agents behave independently of each other, abandoning to the market the coordination of their individual decisions. The implications of this model are extensively presented in the traditional price theory textbooks. Imperfect competition is the paradigm that develops as soon as economic agents interact in a conscious manner, which is the rule when competition takes place amongst a restricted number of agents. In this system, agents act strategically, taking into account the impact of their decisions on competitors' behaviour and on the price mechanism. Such situations commonly arise when firms differentiate their products, erect strategic entry barriers, or exploit the imperfect information of their customers about the price or characteristics of their product. This book explores the theoretical richness of these economic contexts, using some basic tools of game theory. Designed as an ancillary text for graduate students, it not only summarizes the historic contributions made by economic theorists such as Cournot and Edgeworth, but also makes accessible many of the most recent developments in the same field.