SIMM (Software for Interactive Muskuloskeletal Modeling) is a powerful tool kit that facilitates the modeling, animation and analysis of 3D musculoskeletal systems. In SIMM, a musculoskeletal model consists of representations of bones, muscles, ligaments, and other structures. Muscles span the joints and develop force, thus generating moments about the joints. SIMM enables an analysis of a musculoskeletal model by calculating the joint moments that each muscle can generate at any body position. By manipulating a model using the graphical interface, the customer can quickly explore the effects of changing musculoskeletal geometry and other model parameters on the muscle forces and joint moments. SIMM is now used by hundreds of biomechanics researchers to create computer models of musculoskeletal structures and to simulate movements such as walking, cycling, running, and stair climbing.