Expanded HIV screening in healthcare settings, and emergency departments (EDs) in particular, is essential to improve health outcomes and reduce HIV incidence, disparity, and cost. However, implementation of HIV screening programs in EDs remains rare. A primary barrier is the concern that there is insufficient capacity to provide this additional service. It is assumed that providing HIV testing takes away time and resources for providing emergency care, yet the actual opportunity costs that occur from implementing ED HIV screening are unknown. Measuring the impact of different HIV screening approaches on usual ED operations would be a critical advance. There are a multitude of approaches to ED HIV screening and many different types of EDs. Trying to prospectively evaluate the impact of every possible combination is simply not possible. Discrete event simulation (DES) presents an alternative approach. This is a rigorous and efficient method to model complex dynamic systems and evaluate how changes to a system affect performance. The goal of this proposal is to: i) quantify disruptions in ED operations that result from different HIV screening approaches, and ii) identify ways to minimize the disruption that results from conducting HIV screening in the ED. Our team combines expertise in the areas of operational simulation, emergency medicine, and HIV screening. We will create a robust simulation model that incorporates the broad variability presented by EDs and different screening program models. Specifically we aim to: 1) define primary models of ED HIV screening programs, 2) simulate baseline ED clinical operations, and 3) test and optimize the operational impact of ED HIV screening models on ED operations. To obtain robust estimates for simulation parameters, we will perform time-and-motion observations of ED HIV screening activities and usual ED operation in the absence of ED HIV screening in selected sites. This highly innovative project will have rapid and substantial impact by: i) guiding ED HIV screening implementation, ii) spurring innovation in screening program methods, and iii) demonstrating the utility of simulation for implementation research involving any number of healthcare initiatives including clinical practice guidelines, error prevention, and reduction of healthcare acquired infections. More tangibly, this research will have a direct effect on health by accelerating the pace at which HIV screening is widely adopted throughout the specialty of emergency medicine.