RTI International conducted a project for the Centers for Disease Control and Prevention (CDC) that involved expanding a dynamic, compartmental model of HIV that was developed by CDC using Stella software and that contained a single risk group: men who have sex with men (MSM). Under this project, the project team modified the Stella model to add heterosexuals, intravenous drug users (IDUs), and women. However, even after this modification, the Stella model was still unable to easily handle additional model complexity, such as differences in HIV risk across racial/ethnic groups and mixing of HIV risk subpopulations. So, RTI, reconstructed the Stella model using MATLAB to include racial/ethnic differences and incorporate methods that capture the infection risk of each subpopulation and mixing of the modeled subpopulations. The MATLAB model included an optimization component so that strategies to minimize the number of new HIV infections could be identified. RTI also trained CDC's Prevention Modeling and Economics Team (PMET) on the MATLAB model so that PMET staff could run the model and refine it over time.