Voluntary medical male circumcision (VMMC) is a cost-effective HIV-prevention intervention that reduces the risk of HIV acquisition in men by 60%. Although some countries are successfully scaling up VMMC, not all are doing this. When VMMC scale-up experiences are viewed in the context of models for the diffusion of innovation, some important themes emerge. Successful VMMC programs have in common locally led campaigns, a cultural tolerance of VMMC, strong political leadership and coordination, and adequate human and material resources. Challenges with VMMC scale-up have been marked by less flexible implementation models that seek a full integration of VMMC services at public medical facilities and by struggles to achieve geographic parity in access to care. Innovation diffusion models, especially the endogenous technology model, and multiple levels of influence on diffusion--individual males and their sex partners, communities, and health systems--remind us that the adoption of a prevention intervention, such as VMMC, is expected to start out slowly and, as information spreads, gradually speed up. In addition, the diffusion models suggest that customizing approaches to different populations is likely to accelerate VMMC scale-up and help achieve a long-term, sustainable impact on the HIV epidemic.