This book represents the outcome of the Second IWA Leading-Edge Conference held in Sydney, Australia in November 2004. Sustainability is a paradoxical concept. We know we want to protect the environment from human-induced change, yet ecosystems are dynamic, constantly changing and adapting in response to a multitude of factors, the combined effect and subtleties of which are probably well beyond human calculation. Furthermore, our conscious desire to protect the environment - which forces us to think of humans as sitting outside ecosystems - conflicts with the unavoidable fact that we are an unconscious actor within those ecosystems. We must also recognise that the goal of protecting the environment is not a clear-cut objective. Perhaps, because of its complexity and propensity to change, we cannot know what the fully protected environment would look like. Individual preferences too make the conceptualisation of an ideal state impossible; do we strive for an ecosystem in which we play a minor part - barely influencing natural outcomes - or one that is more actively managed and provides for our needs or wants? Neither this volume, nor the conference from which it draws, resolve the paradoxes described above. The papers presented here do, however, provide insight into the innovative thinking and practical projects undertaken across the globe that move us from patently unsustainable conditions to those in which our economic activity, impact on ecosystems and desire for positive social outcomes are in better balance.