There is a consensus that defendants who plead guilty generally receive less harsh sentences than those convicted at trial. However, there is much less consensus on the magnitude of the difference between the sentences imposed at trial and at plea. Theories from legal and criminal justice research have highlighted the importance of the size of the plea discount. Yet the empirical estimation is a very complicated task. Because of the differences in law and local practice, there can be considerable variation in the plea discount among states and counties. Moreover, some practical issues related to the availability of data may also limit the types of research that can be implemented. This article begins by summarizing legal and criminological theories that explain the plea bargaining process, as well as the implication of the size of the plea discount in them. It then reviews the empirical findings of recent studies on the plea discount. It concludes with a discussion of some general issues research on plea bargaining is facing, as well as potential ways to move forward.