The psychology of forgiveness originated from the creative and important work on the development of justice initiated by Piaget in 1932 and extended by Kohlberg in 1969. The scientific study of forgiveness is quite new, having emerged in print in 1989, with an examination of the developmental progression in children’s, adolescents’, and adults’ thinking about the necessary conditions for them to offer forgiveness to another person. In this chapter, the authors first review the definition of forgiveness, followed by this early cognitive work. They then turn to a discussion of the measurement of forgiveness correlates of forgiveness. The practical application of this construct is seen in the development of forgiveness therapy and forgiveness education, which the authors discuss in light of the empirical findings. Future directions for forgiveness studies are considered.