Drawing on theoretical insights from basic psychological research, influences sourced in unanticipated or extra-legal aspects of criminal cases that can influence sentencing decisions are reviewed. Findings from empirical studies of observed disparities in sentencing decisions are summarized, including archival data, observational studies, field research and experimental investigations of simulated sentencing decisions. A criterion for inclusion was that the factor had to exert an influence outside the conscious awareness of the judge. The paper examines the effects on sentencing of some traditional extra-legal factors, such as judge and offender gender, as well as less traditional elements, such as the judge's attitudes and sentencing philosophy, the physical appearance of the offender, unforeseeable consequences of the crime, and reactions to acts of terrorism. Four sources of potential bias are distinguished: (a) features associated with the judge; (b) features associated with the offender, (c) unanticipated or remote consequences of the offence, and (d) contextual information at the time of the sentencing decision that heightens awareness of mortality. The effectiveness of sentencing reforms to reduce disparities arising from unconscious factors is discussed. The purpose of this review is to advance understanding of the psychology of sentencing and increase judicial and public awareness of unconscious biases and disparities in sentencing determinations.