Book excerpt: Principles of Sustainable Development is the component of Encyclopedia of Development and Economic Sciences in the global Encyclopedia of Life Support Systems (EOLSS), which is an integrated compendium of twenty one Encyclopedias. Sustainable Development is a term of differing definitions. Standing alone, the term is abstract and ambiguous. The meaning most often cited is that adopted by the World Commission on Environment and Development: meeting today’s true needs and opportunities without jeopardizing the integrity of the planetary life-support base – the environment – and diminishing its ability to provide for needs, opportunities, and quality of life in the future. This definition may serve as a general principle, but for a guide to action its components sustainability and development must be given substance: what is to be sustained and what developed? Is development essentially economic or material growth, and is sustainability mostly a means to keep economic growth growing? Consequently, should development represent means toward ecologically sustainable ends? The concept of ecological sustainability has been advanced as a restriction on economic development. It follows therefore that principles of sustainable development depend upon how the term is understood and how it is put into practice. Even so the definition of the World Commission on Environment and Development, given the adequate definition of variable needs, provides the most reliable principle for testing the qualitative and ecological sustainability of development proposals. The Theme on Principles of Sustainable Development, in three volumes, deals with the diversity of points of view on this complex subject. These three volumes are aimed at the following five major target audiences: University and College students Educators, Professional practitioners, Research personnel and Policy analysts, managers, and decision makers and NGOs.