Secure supplies of water, food and energy are essential to human dignity and well-being around the globe. In turn, the vitality of these three depends on a thriving biodiversity supported by healthy ecosystems. The complex interdependence among these four factors is known as the Nexus. Global demand for the first three elements is increasing due to population growth and rising per capita incomes in developing countries, with steadily worsening consequences for the fourth of these elements. The four Nexus elements are also coming under increasing pressure from climate disruption: more frequent and severe flooding and storms, droughts, extreme heat, and pest outbreaks. What’s more, Nature’s capacity to moderate these impacts is being steadily eroded by rapid, widespread land-use development and associated pollution. This impending “perfect storm” of increasing demand, decreasing supplies and rapidly changing hydro-climatic conditions throughout the Nexus requires transformative policy responses that encompass economy, equity, social justice, fairness and the environment. This book outlines these challenges and offers a pathway to resolving them.