Abstract In this article we argue that the City Monitor for Sustainable Urban Development in the Flanders (Belgium) acknowledges two kinds of complexities. Firstly, the set of almost 200 SDIs (Sustainable Development Indicators) is positioned in complex and strategic decision-making processes in Flemish cities. In this respect, this learning instrument contains actor-exceeding and policy-exogenous information, which is relevant for governance settings involved in the urban (sustainable) development of their city. The City Monitor is meant to enhance and sharpen the quality of strategic urban debates and, as a consequence, it has to be regarded as only one single element in complex urban decision-making processes. Secondly, the design methodology of the City Monitor also aims at addressing typical tensions brought about by such catch-all terms as urban sustainability. Because of the complex and normative character of the concept we opted for an intensive co-design approach with hundreds of urban stakeholders. The case of the City Monitor shows that this ‘complexity-acknowledging’ perspective and approach can be complementary to more traditional monitoring approaches.