This landmark study forms part of the series UNU Studies on Critical Environmental Regions. The Basin of Mexico exemplifies an endangered environment well on its way to criticality. At the helm of this dangerous course is Mexico City, that massive megalopolis in which dramatic concentrations of all kinds have coalesced to render an already precarious environment supremely capable of andquot; biting backandquot; and posing an immediate and long-term threat to human use, health, and well-being. The extraordinary pace of environmental changes is already depleting natural resources in the region and beyond and may be overwhelming local environmental sinks as well as institutional and societal capacities to cope. An ever-increasing propensity to inhibit ongoing environmental degradation by enlisting imported resources and awaiting technological fixes may well have the effect of increasing overall risk. At the same time, however, the prospect of reversing the trajectory toward criticality is not altogether out of the question and may well be in progress in specific areas. Ezcurra and his colleagues rightly view the region as a laboratory engaged in testing many of the processes under way in the less-developed regions of the world. The outcome of this experiment may signal the fate of other mega-cities across the globe.