Abstract: Object tracking is a crucial subject in computer vision. Due to the variety of the target feature and the complexity of the background, target tracking has been a challenging subject all the time. Especially the tracking under occlusions, is one of the difficulties. How to deal with tracking under occlusion effectively, especially tracking under severely occlusion and totally occlusion, becomes the crucial restrictive factor to the robustness of tracking algorithm. This paper summarizes and classifies the main algorithms of tracking under occlusions home and abroad, expounds the basic ideas of these algorithms, introduces briefly the representative algorithms, analyses the advantages and disadvantages of these algorithms, and the situation that the algorithms fit. At last, necessary discussion is given about tracking under occlusions.