This study examines the extent that people on probation and parole contribute to crime as measured by arrests. Key findings in these four jurisdictions show that the majority of all adult felony and misdemeanor arrests were of people who were not currently under supervision; people under supervision accounted for only 22 percent of total arrests; whereas people under probation and parole supervision accounted for one out of every six arrests for violent crimes, they accounted for one out of every three drug arrests; during a 3.5 year period in which total arrests fell by 18 percent, the number of arrests involving individuals under parole supervision declined by 61 percent and by 26 percent for individuals under probation supervision. The chiefs of the Los Angeles, Redlands, Sacramento, and San Francisco Police Departments commissioned the analysis in 2010. Data were collected from 11 independent agencies, including four local police jurisdictions, county law enforcement and probation agencies, two county sheriffs' departments and the California Department of Corrections and Rehabilitation (CDCR); researchers collected and matched more than 2.5 million arrest, parole, and probation records generated between January 1, 2008 and June 11, 2011.