Water has been explicitly (or sometimes implicitly in its climate relationships) discussed in nearly all preceding chapters. Water links the plot, landscape and governance scales of the three agroforestry concepts (Chapter 1), it is a key determinant of tree growth and adaptations (Chapter 2), relevant traits can be a target of tree domestication (Chapter 3); water is an important component of soils (Chapter 4) and treesoil-crop interactions (Chapter 5). The pantropical analysis of agroforestry (Chapter 6) found climate (and specifically the ratio of rainfall and potential evapotranspiration) to be a major determinant of tree cover on agricultural lands. All the landscape examples dealt with water, through restoration and modification of microclimate (Chapters 7, 8 and 12), through contested land use rights and watershed functions (Chapters 9, 10 and 11). One of the key features of small islands (Chapter 13) is a shortage of freshwater storage, while excess and deficits of water are at the basis of many disasters (Chapter 14). In this chapter we will discuss how the shift in agroforestry concepts (from field/farm-level AF1, to landscape level AF2 and governance level AF3, as detailed in Chapter 1) has interacted with research and contributed to an increased understanding of the way all water-related aspects are interlinked, urgent in the current sustainable development discussion, and open to a wide range of tree and agroforestry- based interventions (with several examples of how such interventions have backfired where understanding was incomplete). Hydrological, ecological, social, economic and policy aspects of trees as part of various land uses in relation to water, are tightly linked (a Gordian knot?). Yet, the relationship between tree cover and human water security is strongly contested1 (Fig. 17.1), with ‘pumps’ versus ‘sponges’ as key features of forests2 and atmospheric recycling as arena of debate.