To make sense of the controversies about Sahelian desertification, we must be able to assess objectively the processes, their location and level of threat to drylands and those who inhabit them. Surprisingly, after the United Nations Conference on Desertification (UNCOD) was set up in 1977, there still are no reliable maps or means of monitoring desertification at the sub-national to global scales, even in the iconic Sahel region. These are the scales needed to formulate institutional policies for prevention and remediation and for Earth system science. While there are a few maps that do extend to larger areas, they mostly have serious shortcomings. This void is partly because suitable metrics have not been available and those that have been proposed can be difficult or impossible to apply over large areas. One of the most basic problems is the lack of comparison with non-desertified sites and an absence of validation, without which statements about more or less desertification are misleading. Furthermore, existing maps are mostly based on subjective assessments by experts, and therefore cannot be applied elsewhere or by different observers, nor can they be used in future for monitoring. Consequently, unsupported statements about the extent and severity of desertification abound. This chapter reviews the existing global maps and sets out principles for more rigorous mapping and proposes methods that adhere to these. Decades of work have improved our understanding of individual components of the desertification processes, but have not answered the three fundamental questions at the regional to global scales: What is degraded? Where does it occur? How severe is the degradation?