While substantial progress has been made in responding to the HIV epidemic, progress in treatment and prevention of HIV may be fragile. This study will assess the ongoing evolution of the HIV epidemic, including whether the epidemic begins to concentrate within high-density networks and if measurable threats, such as HIV drug resistance and shifts in risk perception, will negatively impact epidemic control, providing a data-driven model for how to best respond to this changing epidemic. Results from this study will directly inform American health, investments, programs, policy, and research.