Every day, thousands of defendants, prosecutors, and defense attorneys must make guilty plea decisions, such as whether to accept a plea offer or proceed to trial. Most defendants opt to plead guilty; approximately 95% of state and federal convictions result from guilty pleas. In light of a newly emerging body of research and recent Supreme Court decisions on guilty pleas, this article asks and answers 2 questions: First, who pleads guilty and why? We describe the characteristics of those who are more or less likely to plead guilty, and examine the reasons why individuals plead guilty instead of proceeding to trial, exploring the cognitive, social influence, and developmental factors that underlie decision making. Second, are defendants’ plea decisions valid, in that the decisions are made knowingly, intelligently, voluntarily, and with a factual basis of guilt? That is, do defendants who plead guilty understand and appreciate the conditions and consequences of their pleas, as required by law? Are innocent people induced to plead guilty to crimes they did not commit? We conclude with suggestions to move the field of plea research forward.