Considering global trends such as climate change and resource scarcity, a major challenge of future cities will be to reduce urban footprints. Moreover, cities have to become or remain livable for their inhabitants and offer social and economic opportunities. Thus, reconnecting food production and cities offers promising potential. The diffusion of urban farming reflects a rising awareness of how food and farming can shape our cities. A growing number of urban farming projects exist in and on urban buildings, including open rooftop farms, rooftop greenhouses and indoor farming. These projects are characterized by the non-use of land or acreage for farming activities. We use the term ‘Zero-Acreage Farming’ (ZFarming) to represent these farms. The objective of this paper is to: (1) illustrate and systemize present practices of ZFarming and (2) discuss specific novelties of ZFarming in the wider context of urban agriculture. We analyzed 73 ZFarms in cities of North America, Asia, Australia and Europe using a set of criteria, and developed a typology of ZFarming, complemented by in-depth interviews with pioneers in rooftop farming in New York. The results illustrate that ZFarming generates innovative practices that may contribute to a sustainable urban agriculture. Besides growing food, it produces a range of non-food and non-market goods. It involves new opportunities for resource efficiency, new farming technologies, specific implementation processes and networks, new patterns of food supply and new urban spaces.