This paper introduces in detail the research of visual tracking which is a hot spot currently in the domain of computer vision. Firstly, the applications of visual tracking in three areas including visual surveillance, image compression, and 3-D reconstruction are discussed. Secondly, the state of the art about visual tracking is introduced, especially the main approaches of visual tracking are shown. In order to explain these methods clearly, the problems of visual tracking are classified. Then two ways to research the visual tracking problem are presented. And the visual tracking algorithms are classified into four classes: the area-based methods, feature-based methods, deformable-template-based methods and model-based methods. Finally, from the point of view of control theory, the difficulties of visual tracking are discussed that the algorithms should have robustness accuracy and be fast. Meanwhile, some future directions of visual tracking are also addressed shortly.