Freshwater scarcity is a global systemic risk. Its impacts reach far beyond socio-economic and environmental challenges and influence people’s livelihoods and wellbeing. As water scarcity deepens in arid and overpopulated regions, there is a need to explore water supply options beyond conventional water resources—snowfall, rainfall, river runoff, and easily accessible groundwater—since they already often fall short of meeting the growing water demands. Water-scarce countries need a radical re-thinking of water resource planning and management and, among other options, turn to unconventional water sources for food production, livelihood, ecosystems, and overall requirements—for sustainable development. Such water resources exist ranging from the Earth’s seabed to its upper atmosphere. Securing access to them requires specific technologies and innovations. This introductory chapter takes stock of water scarcity trends, puts forward unconventional water resources as a critical response to global water scarcity, provides insights into linkages with water-related sustainable development, and introduces the book’s parts and chapters.