Since the United Nations Conference on Desertification held in 1977 in Nairobi, much progress has been made abroad on classification and assessment of desertification. Three major steps accompanied with three new definitions of desertification are discussed respectively,which are UNCOD in 1977,“provisional methodology for the assessment and mapping of desertification” in 1984 and the first edition of the World Atlas of Desertification in 1992. Fully representation on type, severity, degree, hazard, indicator of desertification is in the newly released world atlas. Three of them are important, including the global assessment of human-induced soil degradation(GLASOD), the assessment of the status of human-induced soil degradation in South and South-East Asia(ASSOD), and the theory put forward by Moscow State University and Russia Academy of Science. Great differences on the classification and assessment focus chiefly on the evaluation concerning method and indicators. In the GLASOD, severity=degradation degree·extent per polygon, in the ASSOD, severity=productivity change·management level·extent per polygon and RUSSIA=diversity degree·number of main degradation factor. To improve the level of classification and assessment in science and authority, efforts should be taken in establishment of theoretical base of degradation which including determination of temporal and spatial scale.