In this week's PLOS Medicine, Anna Borquez from Imperial College London and an international group of authors developed a mathematical model representing the HIV epidemic among men who have sex with men (MSM) and transwomen in Lima, Peru as a test-case for the effectiveness of HIV pre-exposure prophylaxis (PrEP). The model was used to investigate the population-level impact, cost, and cost-effectiveness of PrEP under a range of different scenarios. The authors found that strategic PrEP intervention could be a cost-effective addition to existing HIV prevention strategies for MSM populations. However, despite being cost-effective, a substantial expenditure would be required to generate significant reductions in incidence of HIV.