Priority prosecution involves identifying serious habitual offenders and putting them on a prosecutorial track that results in a high conviction rate, long sentences, substantial time served, and delivery of the best correctional and aftercare services. Many prosecutors’ offices established priority prosecution units in response to staff shortages and enlarged caseloads. Their efforts involve close cooperation between prosecutors and police, assignment of highly experienced or specialized attorneys, vertical prosecution, coalescing of cases, close supervision by a senior prosecutor, curtailed plea negotiations if they occur, and reduced caseloads. The study of the Philadelphia Habitual Offender Unit aims to determine whether Philadelphia’s selection criteria are effective early warnings of serious habitual offenders or need improvement and whether the unit has reduced the frequency and seriousness of crime to a greater extent than regular prosecution and special prosecution. The study will take place in two phases and will use police and court records and participants’ self-reports.