Recent years have seen a growth of interest in the value of ‘girls’ empowerment’ for international development. When it comes to teenage girls, efforts to ‘empower’ tend to focus on outcomes implicitly linked to their sexuality. Popular discourse on the African teenage girl tends to paint a picture of a person who fundamentally lacks agency over her body and relationships. These narratives pay little attention to girls as sexual subjects. This thesis uses ethnographic methods to explore how teenage girls in Northern Tanzania experience girlhood and sexuality. Insights from the new social studies of childhood and the literature on sexuality in development are drawn upon to move beyond existing approaches to researching sex with young people. Rather than viewing girls as passive victims of oppressive sexual norms, girls’ sexual agency is the focus of the research. Whilst girls in Tanzania are expected to defer to sexual norms which are restrictive and highly gendered, the girls I met sought various ways to negotiate, subvert and resist expectations which limited their agency. Development narratives often problematise girls’ sexuality, yet I found that some girls were able draw upon sexuality and sexual relationships as a way to expand the range of choices available to them. School-based education is frequently presented within international development as a route for the empowerment of girls, but school itself can limit the way in which girls are able to navigate restrictive norms around girlhood and sexuality. This thesis argues that approaches by international development organisations working with girls must both recognise girls as sexual subjects, and pay attention to the role of various socio-economic factors in shaping girls’ opportunities for sexual agency.