Technical Abstract: Arid lands throughout the world, including lands at the border of arid regions, are increasingly subject to human impact, leading to degradation of soils, losses of plant production, and a diminished economic potential to support human populations. Focusing on the human impact and consequent losses in economic potential, we often call these changes "desertification." With the potential for global climate change, however, the definition of desertification and its potential must be expanded. Indeed, the 1992 United Nations’ Desertification Convention defined desertification as "land degradation in arid, semiarid and dry subhumid areas resulting from various factors, including climatic variations and human activities." It is more recently recognized that desertification involves human and environmental drivers but is a symptom evident at regional spatial scales that emerges from degradation at finer spatial scales. Assertions relative to an updated and revised paradigm regarding desertification have been developed. In l981, a group of scientists based in Las Cruces, New Mexico, and associated with New Mexico State University proposed a program of long-term ecological research in the Jornada Basin of southern New Mexico (USA) to gain a better understanding of processes that determine the structure and function of desert ecosystems. To an extent, this book represents both a synthesis of that effort and a benchmark of our progress over the last 23 years. In addition, this book draws on a longer history of research in the Jornada Basin that dates back to the early part of the twentieth century. Both the group and its mission have evolved since 1981, but an initial motivation for our studies was the dramatic, historic records of vegetation change in the Jornada.