"Something there is that doesn't love a wall" -Robert Frost Robert Frost's poem "Mending Wall" captures the contradictory character of our relationships to borders. In the US, common understandings of the Mexico-US border see it as necessary to keep out intruders and establish a barrier between nations, races and social classes. In Mexico some of these same ideas exist, usually expressed in a more critical tone born of an understanding of the asymmetry of power relations between the two countries. The Mexico-US border is a formidable barrier, but people, things and ideas do move across it with lesser or greater ease; water in rivers and aquifers moves across borders because it flows downhill, regardless of the political boundaries of states. Human movements are mediated by kinship, community, state bureaucracy and a host of affective dimensions, often quite positive. In fact, these movements are often spurred by the existence of the border, as people seek advantages on one side or the other. The ceaseless process of crossing borders gives pause to reflect on their constructedness, their functions, and, as Frost put it, what we don't love about them.The essays grouped together in this special section of the Journal of Political Ecology present a Mexican perspective on water in the Mexico-U.S. borderlands. To insure coherence among the articles, the authors were asked to discuss the transnational dimensions of water use and management in the borderlands. The project to collect and present articles of leading Mexican scholars of water in English had its origin in the dissatisfaction of this author with the failure of many Mexican and US scholars studying water to really engage in discussion across the national divide. Of course, there are some who have made this crossing (Evans, forthcoming; Melville and Whiteford 2002; Sanchez Munguia 2005), but scholars of the US West and water historians in particular (for example, Worster 1985;Pisani 1984Pisani , 2002 often ignored the long and important trajectory of Mexican anthropology and history of water (reviewed briefly below), and the existence of strong research on the topic in the northern Mexican borderlands. That scholars on both sides of the national divide shared major research themes and theoretical questions -aridity, hydraulic infrastructure, water law, state formation, capitalism -made this disjuncture even more perplexing. Translating and publishing Mexican water scholars in an open-source journal in English looked like a promising step across the wall that separates academic production in Mexico and the United States.Like many other dimensions of life in the borderlands, the scholarship in the pages of this special section of the Journal of Political Ecology is at the same time oriented by, yet transgressive of, the national-state divide and the ways it has shaped the dynamics of the social use of water in the region, as well as the intellectual effort to understand these dynamics. Water has long been an exceptionally creative and fertile field of study in ...