Capital punishment jurisprudence and practices have evolved throughout US history. Relative to prior eras, the death penalty in the modern era is infrequent, geographically isolated, and has numerous restrictions. This encyclopedia entry reviews the contributions of psychological theory and research to the death penalty phenomenon. After reviewing the characteristics of the modern death penalty process, the entry reviews the role of psychology in legal developments to death penalty eligibility. Psychological research has demonstrated that certain people, such as juveniles, have diminished capacities for rationally understanding their actions and the subsequent consequences. Thus, the Supreme Court has ruled that executing these offenders does not achieve goals of punishment and often runs contrary to contemporary ‘standards of decency’. The entry then describes the local nature of the death penalty, and psychological explanations for why the death penalty is isolated in particular regions, states, and counties. Next, the entry focuses on capital trials. The first of these sections centres on a unique aspect of jury selection in capital cases, ‘death qualification’; the second reviews research into jurors’ comprehension of death sentencing instructions; the third section turns to how individual differences in social, demographic, and psychological variables relate to capital jurors’ decisions; and the fourth reviews research examining interactions among trial participant characteristics and case characteristics. Readers of this entry will gain an understanding of the death penalty’s evolution, the role of psychology in developments to the death penalty, and the contributions of psychological research in explaining how jurors make decisions in capital trials.