The extent of lethal gang violence in Chicago was determined through statistics compiled by the Illinois Criminal Justice Information Authority. The data include detailed information on all homicides in Chicago from 1965 to the present, as well as documentation of the pattern of all gang-related criminal offenses in Chicago from 1987 to 1994. According to the Homicide Dataset, the number of gang-related killings did not exceed 100 in a given year until 1990, but the violence escalated rapidly in the early 1990's; in 1994, 240 gang-related killings were recorded in Chicago. The number of street gang homicides increased almost 30 percent from 1992 to 1993 and increased nearly 45 percent from 1993 to 1994. Much of the violence in the early 1990's was attributable to rivalries among some of the largest African-American street gangs. Violence also increased among Hispanic street gangs. Guns played a prominent role in most gang assaults. The weapon most often used was a handgun, usually a .38 or lower caliber. Beginning in 1991, however, the weapon of choice began to change, and most of the increase in deaths from 1990 to 1994 is attributed to the use of semiautomatic and automatic weapons. To help police and other agencies better counter the gang problem, the Authority developed two tools for identifying and predicting gang violence. One tool is the GeoArchive, a database of community and law enforcement information that can be used to plot gang activity. The other tool is the Spatial and Temporal Analysis of Crime (STAC) software package. The STAC locates clusters of criminal activity by automating such analytical functions as time-of-offense data analysis and manual pin mapping. Together, these tools compose the Early Warning System for Street Gang Violence.