A workshop covering advanced topics in OpenSim was held at Stanford University Wednesday-Friday, January 14-16, 2009. It was an opportunity to learn more about how OpenSim works "under the hood" and get supervised, hands-on assistance with problems participants brought to the workshop. On the first day, a conceptual overview of OpenSim was provided. The second and third days were devoted to working on problems that participants brought to the workshop. For example, participants may have generated simulations from motion capture data collected in their own lab, added arms to their gait model, wrote a new muscle model or controller, or analyzed the results of their simulations. They worked in small breakout groups with OpenSim experts on their specific research problems.