Puberty and menstruation are a fundamental part of the second decade of girls’ lives. Yet many girls in low and middle-income countries know very little about the physical and emotional changes that are part of growing up. This lack of knowledge can reinforce girls’ feelings of fear, shame or embarrassment – especially when coupled with sociocultural norms that deem girls and women 'impure' or 'dangerous' during menstruation. Risks created by girls’ lack of information about their own bodies are amplified by the very real threats to which many are exposed once their bodies begin to mature, including child marriage, social isolation, school dropout, and sexual violence. Recognising that even in 2017, a perfectly normal physiological process is truncating adolescent girls’ trajectories, there is an urgent need – as policy-makers, development practitioners and thought leaders gather at the London Family Planning Summit – to pay greater attention to understanding what girls know about growing up and how they can be supported through healthier transitions. This agenda for action is based on a rapid evidence review undertaken as part of the Gender and Adolescence: Global Evidence (GAGE) research programme. It was motivated by two research questions: 1) What do we know about young adolescent girls’ knowledge and experiences related to puberty and menstruation? and 2) What are the household, community and/or programmatic factors that shape that understanding and those experiences?