Histogram Equalization is a simple and effective technology in image enhancement, and it is widely used in medical image processing, computer vision and remote sensing image. When selected different equalization regions, histogram equalization can be divided into global and local histogram equalization method. While the equalizations choose different pixel area, it will be divided into traditional histogram equalization, bi-histogram equalization, recursive mean-separate histogram equalization, and even multi-histogram equalization. Histogram equalization also can be divided into histogram equalization based on space domain and histogram equalization combined with frequency domain in equalization objects. From the channel numbers of equalized image, it will be divided gray image equalization and color image equalization. This paper systematically summarizes the histogram equalization methods mentioned above, describes the evaluation index to measure equalization effect, and look into the distance of further research direction in histogram equalization technology.