This chapter briefly introduces the nexus approach, focussing on water issues. It is not an attempt to summarise the vast literature on the topic but rather to illustrate the concept by presenting selected examples of each element of the nexus and their interlinkages (water–energy, energy–food, and food–water) from a global perspective. We also present three case studies that use the nexus approach in Andean countries to analyse the trade-offs between the elements that arise when designing sustainable development strategies. Two of the case studies use a water–energy–food–environment modelling toolkit that is applied to agricultural development in Peru and Ecuador. The third case study shows a water–food–energy model that links water used in agriculture and hydroelectricity to evaluate climate change adaptation strategies.