Control of chlamydia infections in young people is a public health priority reflected in a National Screening Programme for 15-24 year olds. However, effective screening interventions may exacerbate health inequality if uptake is different across social groups. This thesis aims to i) examine the prevalence and socioeconomic patterning of chlamydia infections, and ii) conduct formative research to inform a Peer-Supported Chlamydia Screening intervention. I conducted a systematic review of chlamydia prevalence in Europe, North America and Australia and found strong evidence of heterogeneity in prevalence estimates among 16-24 year olds and mixed evidence of an association between measures of social disadvantage and higher infection risk. Chlamydia prevalence and social variation at age 17 were further examined in the Avon Longitudinal Study of Parents and Children birth-cohort. After missing data analysis, chlamydia prevalence was 1.4% (0.8, 2.0); 1.7% (0.8,2.6) in women; 1.0% (0 .3, 1.7) in men. Infection was strongly associated with several measures of social disadvantage. Using grounded theory approaches, in-depth and key informant interviews and non-participant observation in further education colleges, the formative study revealed that a peer-screening intervention could build on existing social phenomena such as frequent and open conversations about sexual health, sharing information and experiences, moral and emotional support and humour. The majority of interview participants indicated that they would be willing to participate in the intervention. The intervention may promote uptake of screening through four mechanisms: 1. acknowledging personal relevance of screening; 2. increasing perceived benefits; 3. reducing perceived costs; and 4. providing opportunities for spontaneous testing. The acceptability and effectiveness of the intervention may be enhanced by offering a choice of self-test kits or cards, wider promotion of pro-screening norms, and attractive and humorous promotional materials. No evidence was found to suggest that the intervention would be less acceptable amongst the socially disadvantaged.