The proportion of the world's population in cities has increased from 17 percent in 1950 to 50 percent in 1994 and is expected to reach 69 percent by 2025. Fifty-eight of the 100 largest metropolitan areas in 1990 were located in developing countries. Cities have become breeding grounds of crime and insecurity in both industrialized and developing countries, and the failure to keep crime to a minimum is costly to both cities and countries. Revictimization of the same small proportion of the population is an urban trend and suggests that crime prevention measures need to be in place very soon after victimization and that temporary prevention measures covering the high-risk period after victimization may be an effective and efficient means of preventing crime. Initial steps taken by national governments to support safer cities have included legislation; prevention councils; the encouragement of research, evaluation, training; and initial financing. Cities are becoming the focal point for crime prevention. Programs include community crime prevention councils, school crime prevention programs, and situational crime prevention projects to reduce burglary. Several factors are crucial to success. The International Center for the Prevention of Crime was established in 1994 and will work with the crime prevention and criminal justice program of the United Nations to make promising practices accessible and develop regional plans. Figure, tables, photograph, crime prevention guidelines from the United Nations Economic and Social Council, and footnotes