Cities play a prominent role in our economic development as more than 80 % of the gross world product (GWP) comes from cities. Only 600 urban areas with just 20 % of the world population generate 60 % of the GWP. Rapid urbanization, climate change, inadequate maintenance of water and wastewater infrastructures and poor solid waste management may lead to flooding, water scarcity, water pollution, adverse health effects and rehabilitation costs that may overwhelm the resilience of cities. These megatrends pose urgent challenges in cities as the cost of inaction is high. We present an overview about population growth, urbanization, water, waste, climate change, water governance and transitions. Against this background, we discuss the categorization of cities based on our baseline assessments, i.e. our City Blueprint research on 45 municipalities and regions predominantly in Europe. With this bias towards Europe in mind, the challenges can be discussed globally by clustering cities into distinct categories of sustainability and by providing additional data and information from global regions. We distinguish five categories of sustainability:
(1) cities lacking basic water services,
(2) wasteful cities,
(3) water-efficient cities,
(4) resource-efficient and adaptive cities and
(5) water-wise cities. Many cities in Western Europe belong to categories 3 and 4.
Some cities in Eastern Europe and the few cities we have assessed in Latin America, Asia and Africa can be categorized as cities lacking basic water services. Lack of water infrastructures or obsolete infrastructures, solid waste management and climate adaptation are priorities. It is concluded that cities require a long-term framing of their sectoral challenges into a proactive and coherent Urban Agenda to maximize the co-benefits of adaptation and to minimize the cost. Furthermore, regional platforms of cities are needed to enhance city-to-city learning and to improve governance capacities necessary to accelerate effective and efficient transitions towards water-wise cities. These learning alliances are needed as the time window to solve the global water governance crisis is narrow and rapidly closing. The water sector can play an important role but needs to reframe and refocus radically.