Mapping and other information-collecting and -ordering techniques, usually aimed at formal police data, can also be used to good effect to capture and organize these experiential assets. In this study, a working group was assembled of Harvard University researchers, officers from the Boston Police Department's Youth Violence Strike Force, probation officers covering high-risk neighborhoods, and city-employed gang-mediation "street workers." The group estimated the number and size of the city's gangs; mapped their turf; mapped their antagonisms and alliances; and classified 5 years of youth victimization events according to their connection (or lack thereof) to this gang geography. These efforts provide an overview of Boston's gang turf; an estimate of gang involvement in high-risk neighborhoods; a sociogram of gang relationships; and an estimate of Boston gangs' direct contribution to youth homicide victimization. Figures, tables, notes, references