Biophysical and other ‘outer limits’ of food, land, water, climatic change, stratospheric chemistry, energy, hazardous substances, non-fuel minerals, human stress, and social and ecological stability, raise fundamental questions about present trends in management methods and in global organization. The diverse outer limits surveyed in this paper reflect complex, poorly perceived, and often unsuspected, interconnections between numerous biological and geophysical processes, many of which are obscure or still unknown. Our lack of predictive power, let alone of quantitative understanding, implies a need to treat essential life-support systems with great caution and forbearance, lest we erode safety margins whose importance we do not yet appreciate.Even those outer limits which now seem remote are relevant to present policy, as their timely avoidance may require us to discard otherwise attractive short-term policies in favour of others that offer less immediate advantage but that retain options which may be needed later. Such alternative policies may have to rely more on social than on technical innovation in order to address underlying disequilibria rather than merely palliating their symptoms. Moreover, some outer limits are sufficiently imminent, or require such long lead-times to avoid, that fundamental changes in policy, in institutions, and in the degree of global interdependence, seem necessary if we are to live to enjoy some of the later and more interesting limits to human activity.