Cities in the global South are confronting unprecedented challenges to urban sustainability and equitable development, particularly in the realm of water provision. Nearly 1.5 billion people worldwide suffer from a lack of safe access to drinking water and sanitation -an increasing proportion of whom reside in cities. Meanwhile, in the gaps of the grid, a diversity of water harvesting and reuse techniques, infrastructures, and institutional arrangements has emerged to provision poor households. Despite the burgeoning presence of the informal water sector, little is known about its institutional character, environmental impact, or relationship with state provision and private supply. Drawing on qualitative and quantitative data collected during nearly 13 months of fieldwork in Tijuana, Mexico, this dissertation queries how informal water use is managed, whether informal water use constitutes an alternative economy and sustainable environmental practice, and to what degree informal water use redefines urban space and alternative development possibilities. Findings reveal that: 1) despite historical efforts in Mexico to federalize and centralize the control of water resources, state action opens 'gaps' in the hydrosocial cycle, and informal institutions manage these 'extralegal' spaces; 2) informal water use is widespread across socioeconomic levels in Tijuana, predominantly managed by household-based institutions, and conserves a surprising degree of municipal water; and 3) the spatiality of contemporary water infrastructures and economies is highly diverse-ranging from bottled water markets to non-capitalist, self-provisioning greywater reuse-and is in fact constitutive of 'splintered urbanism' and alternative modes of development.