Urban sustainability is today commonly considered to be a crucial determinant of global sustainable development, and indicator-based assessments represent a popular means of operationalizing this complex concept. An important aspect of these assessments concerns the elaboration of accompanying conceptual frameworks. Despite the pivotal role of such frameworks, they are often developed in an unstructured manner, without consideration of their potential purposes and impacts, and without awareness and evaluation of available options. Here, we discuss this topic based on both theoretical considerations and the analysis of the conceptual frameworks of 67 urban sustainability indicator initiatives. The discussion makes two contributions: First, we provide guidance for assessment developers for leveraging the potential of conceptual frameworks. Second, considering the analyzed frameworks as de facto definitions of urban sustainability allows us to advance understanding of the meaning of this complex concept, and to critically discuss the conceptual shortcomings that remain in current assessment practice.