Urban growth and shrinkage constitute the overall pattern of growing urbanization across the globe. Studies on urban vacant land (UVL) are few, and have proved to be mainly rudimentary and subjective. This paper first presents the definition of UVL based on bibliometric analysis. Typology, morphology, proximate causes, and the multiple functions of UVL are then analyzed at parcel, transect, city, and national levels based on an international review. Results show that UVL can be categorized by land cover, land usage, and land ownership. Worldwide, UVL has been widespread and extensive. For example, the occurrence probabilities of UVL in the cases of Guangzhou and New York are 8.46%-8.88% and 3.17%-5.08%, respectively. The average vacancy rate of residential land amounts to 11.48% in 65 U.S. cities. Generally, UVL shows fragmentation and irregular shape, and significant spatial differences exist at parcel, transect, city, and national levels. Proximate causes, such as excessive land division, irregularly shaped land parcels, decreases in resident population, deindustrialization, land speculation, insufficient investment, and environmental concerns, can all result in UVL. Currently, UVL has become a gray area of social, economic, and ecological space. However, it can also be considered a potential resource for enhancing urban sustainability. Policy implications to promote urban sustainability using monitoring, control, and differential revitalization of UVL are presented.