POP is a strategy for addressing persistent problems that require police attention. POP uses scanning, analysis, response, and assessment. In this approach, officers not only make arrests but also analyze the areas, times, people, drugs, and conditions that underlie persistent crime problems. The U.S. Justice Department's Bureau of Justice Assistance has funded pilot programs, launched in 1988, that have tested how effective the POP concept can be in changing the conditions associated with drug crimes. Three of the five funded projects -- Philadelphia, San Diego, and Tulsa -- are described in this article. The Tulsa program effectively addressed drug trafficking and associated crimes in North Tulsa's low-income housing projects. The Philadelphia POP effort targeted three high-rise public housing towers and neighborhoods undergoing gentrification. The San Diego experience provides two classic examples: one in how problemsolving can successfully change a neighborhood and a system and a second in how it can dwindle into years of frustration.