This paper reviews quantitative and qualitative evidence on adolescents' sexual and reproductive health from a number of developing countries and, where possible, highlights findings for boys and girls aged under 15 years in those studies where findings are disaggregated by age and sex (which is not always the case). The evidence is sporadic, however, and is by no means representative of all developing countries. The review is organized around six interwoven processes that mark girls' and boys' passage through adolescence, with an emphasis on the early years: status transitions among young adolescents in the context of age-graded standards of children's and adolescents' rights; processes of sexual, reproductive and cognitive maturation; the acquisition of sexual and reproductive knowledge and skills; gender socialization and the influence of sexual and other norms; the shifting terrain of opportunities, risks and protective factors; sexual and reproductive behaviours and their consequences. The concluding section brings together a number of research topics and approaches identified throughout the paper that could form the basis of a coordinated multi-country research agenda for young adolescents' sexual and reproductive health and rights.