By introducing shifting factors into sliding surfaces, shifting sliding mode control (SSMC) is defined. Sufficient existence conditions of SSMC are proved. Based on SSMC, output tracking, in which perfect tracking is achieved in a finite time under matched and bounded unmatched uncertainties, and optimal LQ regulation under matched uncertainties are considered. SSMC makes it possible for the design of control law (to induce sliding motion) and that of system performances to be completed separately. This gives designers one more freedom in improving system performances in sliding mode control setting.