Adolescence is a time of many transitions and a time to acquire the skills for adult life. Although skills can still be acquired in adulthood, adolescence is a time to lay a foundation of education, financial skills, positive health behaviors, and critical thinking. For many girls in the developing world, the opportunity to move freely in the community becomes limited at the onset of puberty. This may be a well-intentioned protective measure, but the effect can be to limit girls’ opportunity to form strong social networks, gain requisite skills, and learn how to be full members of their community. The Population Council has developed a number of “safe spaces” programs that address the needs of vulnerable adolescents, particularly girls. These programs all build on the key elements of a safe physical space, a mentor, and a friendship or social network for the girls. Promoting Healthy, Safe, and Productive Transitions to Adulthood Brief No. 39 explores what safe spaces are, why social or friendship networks are important, the role of mentors, who the girls are, and lessons learned.