Sustainable development is introduced in the article as an ethos and post-modern idea of progress, i.e. an ideal for betterment of humankind. A sine qua non for the ideal is that ecological sustainability is made a quality standard for human economic, social and material welfare. Necessary conditions of ecological sustainability are derived. The theory is based on mathematical elaboration of the self-evident identities between the total environmental stress (TES) and the basic indicators of economic, technological and social development. Explanatory power of the theory is demonstrated with new important concepts and formula.