The Baltimore City Public School System (BCPSS) is one of the first urban districts in the country to undertake large-scale high school reform, phasing in small learning communities by opening new high schools and transforming large, comprehensive high schools into small high schools. With support from the Bill & Melinda Gates Foundation, a dozen local foundations, and school reform developers and technical assistance providers, BCPSS opened 18 small high schools between 2002 and 2006. Using data from surveys, administrative records, and site visits, we examine the implementation and outcomes of these reform efforts. Although BCPSS high schools met many obstacles while implementing the reform, including continual leadership turnover and limited resources, students and staff in the small learning communities reported more positive social and academic environments than their counterparts in the district's comprehensive high schools. In addition, students in small, newly created high schools demonstrated positive academic outcomes (e.g., higher test scores, engagement) relative to students in the other schools in the district. (Contains 3 figures, 11 tables, and 7 footnotes.)