Background: Many HIV prevention interventions have been implemented in India but there is very limited data on costs and effectiveness of large scale HIV prevention interventions targeting high risk groups over time. Most of the existing studies deal with service level resource needs. Moreover, none include the costs of supporting the scale-up above the service level, even though these programs may require substantial initial investment in capacity constrained environments. Objectives: The present study in the southern Indian states looks at estimating the program costs at different organisational levels and specific project level costs and outcomes for different high risk groups and specific interventions like community mobilization and explore the HIV prevention costs by scale and typology over time. Methods: Cost data was collected from 138 NGOs in 64 districts and detailed cost analysis for 23 districts in the four states based on the UNAIDS Costing Guidelines for HIV Prevention. Costs were collected using an ingredients-based costing methodology including both the financial and economic costs. The program data from four states (Karnataka, Andhra Pradesh, Tamil Nadu and Maharashtra) was analysed which is unique in the sense that none of the other studies are of this scale and from India. Results: Unit costs per person reached fell over time, suggesting economies of scale. In addition, the variations found in unit costs between NGOs also suggest efficiency gains with program maturity. Costs structures change substantially over time as HIV prevention programs scale up. Above service level costs are considerable, but usually not measured in cost and efficiency analyses. These costs are essential to consider when scaling up HIV prevention. Conclusions: This thesis would add to the limited evidence base on costs of large scale HIV prevention interventions globally and provide better estimates of costeffectiveness of targeted programs for high risk groups.