Alleys are enigmatic, neglected features of the urban fabric. In this paper we explore the distribution, physical features, activity patterns, and resident perceptions of alleys in one major US city, Los Angeles, California. We do so through an integrated mixed-methods strategy involving participatory research with community-based organizations, spatial analysis, physical audits and behavioral observation of alleys, and focus groups. Results show that most alleys in Los Angeles are underutilized and walkable, quiet, and clean, although they can be, and are often perceived as, dirty and unsafe. Alley density is greatest in park-poor, low-income Latino and African-American neighborhoods. Alleys represent unrealized community assets that could be transformed by urban planners and managers into ‘green infrastructure’ to simultaneously offer multiple ecological, economic, and social benefits—including urban walkability and mobility, play space and green cover, biodiversity conservation, and urban runoff infiltration—and thereby to contribute to a more sustainable urbanism.