The vehicles used to explore the Martian surface require a high degree of autonomy to navigate challenging and unknown terrain, investigate targets, and detect scientific events. Increased autonomy will be critical to the success of future missions. In July 1997, as part of NASA's Mars Pathfinder mission, the Sojourner rover became the first spacecraft to autonomously drive on another planet. The twin Mars Exploration Rovers (MER) vehicles landed in January 2004, and after four years Spirit had driven more than four miles and Opportunity more than seven miles-lasting well past their projected three-month lifetime and expected distances traveled. The newest member of the Mars rover family will have the ability to autonomously approach and inspect a target and automatically detect interesting scientific events. In fall 2009, NASA plans to launch the Mars Science Laboratory (MSL) rover, with a primary mission of two years of surface exploration and the ability to acquire and process rock samples. In the near future, the Mars Sample Return (MSR) mission, a cooperative project of NASA and the European Space Agency, will likely use a lightweight rover to drive out and collect samples and bring them back to an Earth return vehicle. This rover will use an unprecedented level of autonomy because of the limited lifetime of a return rocket on the Martian surface and the desire to obtain samples from distant crater walls.