This thesis aims to describe the uptake and coverage of HIV testing and counselling (HTC) services in a community cohort study in rural Tanzania between 2003 and 2010, and to investigate the impact of HTC on changes in sexual risk behaviour and HIV incidence. Paper A uses data from three HTC services (community outreach HTC (CO-HTC), walk-in HTC (WI-HTC) and antenatal HTC) linked to the community cohort data to compare the characteristics of services users, and found that while WI-HTC was most likely attract HIV-positive individuals, the overall proportion of infected persons diagnosed was greatest at CO-HTC. Rates of repeat testing are important to understand given potential HIV treatment as prevention approaches. Paper B found that small proportions of cohort participants repeat tested between 2003 and 2010, although this improved over time. Paper C presents a quantitative analysis of the impact of CO-HTC on changes in sexual behaviour and HIV incidence, and found moderate associations between HTC use and reductions in some risk behaviours among HIV-negative participants, but no impacts among HIV-positive individuals or reductions in HIV incidence, possibly as a result of small sample sizes and a declining background incidence in the study area. Paper D presents findings from a qualitative study exploring the effectiveness of HIV prevention counselling messages, which showed that relationship dynamics constrained the extent to which HIV-negative women felt able to control their HIVrelated risk, and imbalanced client-counsellor interactions limited communication during counselling sessions. Overall, the findings from the thesis reveal that provision of different HTC models increased the uptake of services, but the proportions of individuals repeat testing were low and there was limited evidence for an impact of HTC on sexual risk reduction. Future research should explore the effectiveness of different HTC modalities in encouraging repeat testing among high risk HIV-negative individuals, influencing sexual behaviour change and linking HIV-positive people to care and treatment.