Abstract. More than 30 years after HIV was first identified as a disease, with disastrous consequences for many subpopulations in most countries and for entire populations in some African countries, it continues to occupy centre stage among the world's many global health challenges. Prevention still remains the primary long-term focus. New biomedical tools such as pre-exposure propyhlaxis (PrEP) and treatment hold great promise for select groups such as key populations (KPs) who are critical to transmission dynamics, and serodiscordant couples. Programs delivering these new tools will need to layer them over existing services, with potential modifications for increased and sustained engagement between health services and beneficiaries owing to the nature of the interventions. Avahan, an HIV prevention intervention for KPs in six states in India, achieved population-level impact with conventional prevention programming, which, however, required high program-beneficiary engagement. Avahan's implementation strategy included articulating clear service definitions and denominator-based targets; establishing routine data systems with regular, multilevel supervision that allowed for cross-learning across the program; and developing a cadre of frontline workers through KP peer outreach workers who addressed structural issues and provided viable and sustainable mechanisms for sustained interaction between health services and KPs. This basic prevention implementation infrastructure was used to expand clinical services over time. Many of the lessons from programs such as Avahan can be applied to KP programs that are expanding service scope, including PrEP and treatment.