Multi-source image fusion is widely used in various visual fields, which integrates the feature information of multiple images into one image.In the past decades, the methods of multi-source image fusion have been increasing, but they still cannot fully meet the needs of users.In order to design a fusion method with excellent performance in terms of accuracy, robustness and efficiency, it is necessary to comprehensively and systematically review and analyze the classic and the latest technologies so as to find new breakthroughs.Following the process of multi-source image fusion, the classification, framework and some methods of image fusion are introduced, and a new classification system of image fusion methods is proposed from a different perspective.Then, a comprehensive summary is made on the fusion evaluation methods.Finally, several applications of image fusion are presented, and the future development trend of multi-source image fusion is prospected.