A dramatic rise in homicide in the latter half of the 1980s peaked during the 1990s and then declined at an equally dramatic rate. Such trends in homicide rates can be understood only by examining rates in specific age, sex, and racial groups. The increase primarily involved young males, especially black males, occurred first in the big cities, and was related to the sudden appearance of crack cocaine in the drug markets of the big cities around 1985. This development led to an increased need for and use of guns and was accompanied by a general diffusion of guns into the larger community. The decline in homicide since the early 1990s has been caused by changes in the drug markets, police response to gun carrying by young males, especially those under 18 years old, the economic expansion, and efforts to decrease general access to guns, as well as an increase in the prison population and a continued decline in homicide among those over age 24. The lessons learned from the recent homicide trends and the factors associated with them have important implications for public health and the criminal justice system.