The theme through much of the book is that of 'sustainable development'. At its simplest, we take this to imply a stance which raises the political and ethical profile of natural and heritage environments as we come to the end of the twentieth century. Sustainable development thinking reflects both our greater understanding of what natural environments do for us as inhabitants and users of those environments, and stresses the uncertainty that results from our continuing ignorance of the many other ways in which we depend on life-support systems.