The publication groups together the results of investigations of the process of desertification by twenty international specialists. It represents a thorough report on the status of our knowledge of the cause and effects of degaradation of arid lands worldwide. An understanding of the physical layout of arid lands and the nature of processes that initiate changes therein is also fundamental to thoughtful utilization of these lands for the benefit of mankind. Emphasis is placed on the regions that are chronically affected by drought in North Africa, particularly in the Sahel.