Young people's reproductive health is a growing policy and research concern, both because of high levels of disease and unwanted pregnancy in this group, and because of a desire to improve less tangible aspects of health such as psychological well-being. To design effective and targeted interventions to improve reproductive health, we need to know both how and why young people behave sexually, and what types of programmes might lead to behaviour change. Little is known about either topic. This thesis uses a qualitative approach to examine sexual behaviour among young people and focuses on a low-income area of Mexico City. The study analyses the personal, social, and physical contexts of sexual activity among young people, the ways activity is explained and justified, and the processes of decision-making about sexual behaviour. Concepts of risk and vulnerability, their variation with context, and their effects on behaviour are explored. In addition, sources and ranges of meanings (individual and shared) of sexual activity and sexuality are investigated, as is the relationship of these to behaviour. A sexual health programme, Mexfam's "Gente Joven" - a peer-led, outreach programme - is also studied, with the aim of understanding how such a programme might affect young people's actions. An interaction-orientated theoretical approach is used. Three social levels are considered: the macro, the individual, and the dyad. Despite not using an overtly gender-orientated perspective while collecting data for this project, it was impossible to analyse the final interviews without gender being considered as a major explanatory element. Stereotypical gender roles where men are controlling and women controlled did not provide a useful framework to interpret the data, however, and a more complex picture emerged with women seeming to have a certain flexibility to reinterpret and transform social rules, and men being more restricted than they first appear. In the final part of the thesis, the processes by which Mexfam's "Gente Joven" programme might affect behaviour are discussed, in the light of the findings from the first part of the study.