The most important concept in current environmental thinking is "sustainability". Environmental policies, economic policies, development, resource use--all of these things, according to the consensus, ought to be sustainable. But what is sustainability? What is its ethical foundation? There is little consensus about how these questions ought to be answered. Surprisingly, almost nothing has been done to justify sustainability as an ethical constraint. In this article, the author's aim is to investigate whether that can be done. He argues that the chief conceptions of sustainability in the environmental literature are not themselves sustainable. They often have innocuous interpretations that are plausible but do little to advance the environmentalists' or any other particular agenda. Their more radical interpretations, however, lack ethical foundations; they face obvious counterexamples when applied to individual lives and communities. There is no reason to take any of them as criteria that any environmental, economic, developmental, or resource management policy must meet, or even as goals toward which any such policy ought to strive. (Contains 38 footnotes.)