Mechanical parts often move, not smoothly, but in jerks known as stick−slip. Stick−slip motion may be regular (repetitive or periodic) or irregular (erratic or intermittent). In the case of frictional sliding, stick−slip can have serious and often undesirable consequencesresulting in noise (chatter), high energy loss (friction), surface damage (wear), and component failure. We review the origins of stick−slip friction and present new experimental results on model surfaces that clarify its different origins, its dependence on experimental conditions or “system parameters”, and how stick−slip can be controlled in practical situations.