As a lexical concept, slum has been widely criticized by twenty-first-century researchers, but the formulation and spread of the concept have profoundly altered actual cities in many parts of the world since the early twentieth century. Examining the discursive history of the slum concept demonstrates the contribution literary studies focused on the city can make to urban history. Urban historians concerned with areas labeled as slums would benefit from problematizing the concept of slum as well as from establishing comparative histories of stigmatized urban zones in a planetary context. Such work leads to a definitional challenge in which undesirable conditions do need labeling in some way, but the challenges and materialities of different cities on different continents are also all unique and potentially damaged by the application of an overarching tag such as slum. In various ways, the contributions to this special feature all address the foregoing issues.