Abstract: A dynamic model is presented for omni-directional wheeled mobile robots, including wheel/motion surface slip. We derive the dynamics model, experimentally measure friction coefficients, and measure the force to cause slip (to validate our friction model). Dynamic simulation examples are then presented to demonstrate omni-directional motion with slip. After developing an improved friction model, compared to our initial model, the simulation results agree well with experimentally-measured trajectory data with slip. Initially we thought that only high robot velocity and acceleration governed the resulting slipping motion. However, we learned that the solid material existing in the discontinuities between omni-directional wheel rollers plays an equally important role in determining omni-directional mobile robot dynamic slip motion, even at low rates and accelerations.