"Sustainable development" has become one of the most widely expressed goals among the environmentally aware, particularly since the Brundtland Report and after Agenda 21 was agreed at the UN conference in Rio de Janeiro in 1992. However, the wide agreement on the importance of sustainability is not matched by any consensus on the meaning of the term or its practical implications for real engineering projects. To improve the management of the water resources of the world it is no longer enough for "sustainability" to be some vague ethical guiding principle: the concept must be clarified operationally. This text is intended to help in this process of definition. The 20 papers included were reviewed, edited and selected from those submitted in response to a call for papers in the summer of 1996. They examine sustainability at the conceptual level and as expressed in new and planned schemes of water and wastewater management (largely though not exclusively in a municipal context).