The chapter starts with an introduction to game theory in biology, describing its overall aims. The basic concept of frequency dependence is then presented, together with a number of illustrative biological examples. Next, the modelling approach is outlined, emphasizing that the theory aims to predict phenomena by seeking stable evolutionary endpoints. The scope and challenges of the field are described in the setting of the history of ideas that have been important for the theory, summarizing past successes as well as long-standing questions that are likely to require further development of the theory. The chapter ends with an overview of the main issues dealt with in the book, including the challenges that are taken up. These include taking into account the co-evolution of traits, exploring the consequences of variation, and the modelling social interactions as games over time. In particular for the latter, models that include behavioural mechanisms are likely to be essential for the success of game theory in biology.