The prevalence and seriousness of gang problems have fluctuated over time, with gang activity escalating during some periods and diminishing during others. The last three decades of the 20th century were characterized by a major escalation of youth gang problems throughout the Nation. This report presents detailed information on the numbers and specific identities of gang problem localities, the size of these localities, rates of growth, and location by State and region of the cities, towns, villages, and counties that reported gang problems between the 1970’s and late 1990’s. By the late 1990’s, 3,700 identified localities in the United States, totaling the highest number ever reported presence of gang problems. In the 1970’s, 19 States reported gang problems; by the late 1990’s, all 50 States and the District of Columbia had reported gang problems. The States with the largest number of gang-problem cities in 1998 were California, Illinois, Texas, Florida, and Ohio. Nationwide, there was a substantial decrease in the concentration of gang cities in the higher ranking States as gang problems continued to spread to new States. The regional location of gang cities changed radically during the three-decade period. In the 1970’s, the West ranked the highest in the reported number of gang cities, and the South ranked the lowest. In 1998, the South ranked second. In the late 1990’s, there were approximately 200 cities with populations of 100,000 or more, and every one of these large cities reported youth gang problems. Gang problems however, were not confined to large cities. One of the best documented developments of this period was a striking increase in the growth of gang problems in the Nation’s smaller cities, towns, and villages. Reasons for the striking increase in the number of gang problem localities are discussed under seven headings: drugs, immigration, gang names and alliances, migration, government policies, female-headed households, and gang subculture and the media. An analysis of projected growth rates of gang problem cities provides a basis for predicting future trends in the number of gang cities. The data provides support for a prediction that the rate of growth that prevailed in the later 1990’s will decrease in the early 2000’s and a prediction that the actual number of gang localities will decrease. Appendices