Diagnosis refers to some state of the person. Classification refers to the allocation of persons to classes such that persons in each class are similar. Diagnoses may be useful in classification, and prediction refers to classification that provides an assessment of a person's expected future behavior. In the pretrial phase, diagnosis, classification, and prediction bear upon delinquency prediction, detention, pretrial release, police and prosecutorial decisionmaking, and competency to stand trial. Equity in sentencing is a statistical concept, and its investigation must rely on the concept of classification. As decisions become less variable with respect to a given classification of offenders, they may be said to be more equitable. In corrections, diagnosis, classification, and prediction have a bearing on confinement and probation placements and parole decisionmaking. Currently, available methods of diagnosis, classification, and prediction are inadequate to provide useful guidance to law enforcement, judicial, and corrections decisionmaking at every stage, but many useful applications exist and promising improvements are imminent. Seventy-three notes are provided.