Thresholding is one of the most widely used image segmentation methods because of its concision and effectivity. Many researchers have followed this technology with great interest and a lot of research findings have been published. Twenty years ago, the first author made a stage review on research progress of thresholding within 1962—1992. But at the present time, thresholding technology has made a big advance and new ideas have been proposed constantly. This paper aims at summarizing and classifying the commonly used hresholding methods in recent twenty years including brand new methods and improvements of the classical methods. Basic thoughts or criteria are given and characteristics and applicable conditions of these methods are clarified. It is hoped that this summarization can provide some thoughts and inspirations to the thresholding researches in future.