Today, a quarter of the global population – 2 billion people – use unsafe drinking water sources. Half of humanity – 3.6 billion people – live without safely managed sanitation. Over 80% of wastewater is released to the environment without being treated or reused. Almost three quarters of all recent disasters are water related, having caused economic damage of almost US$700 billion in the past 20 years. With growing populations, increasingly water-intensive agriculture and industries, and worsening climate change, guaranteeing a well-functioning water cycle for all people everywhere is essential to ensure human health, environmental integrity and a sustainable, equitable future. At the same time, through its close inter-linkages with climate, energy, cities, the environment, food security, poverty, gender equality and health, water can be an accelerator of sustainable development across our societies. ​ Access to water and sanitation is a human right. A well managed water cycle underpins progress across the 2030 Agenda for Sustainable Development, particularly in relation to hunger, gender equity, health, education, livelihoods, sustainability and ecosystems.​ Water is also at the heart of adaptation to climate change, serving as the crucial link between the climate system, human society and the environment. Without proper water governance, there is likely to be increased competition for water between sectors and an escalation of water crises of various kinds, triggering emergencies in a range of water-dependent sectors. ​ ​The physical world of water is closely bound up with the socio-political world, with water often a key factor in managing risks such as famine, epidemics, inequalities and political instability.