A unique project by Mistra Urban Futures to test the draft targets and indicators of the proposed Urban Sustainable Development Goal (Goal 11) in five diverse cities in Europe, Africa, and Asia revealed numerous complexities and differences in data availability, potential accessibility, and relevance. Deploying the findings from Kisumu and Cape Town, we highlight the particular challenges posed by widespread urban informality. Similar issues apply across the global South. The targets and indicators rely on official/formal data, which are often of questionable reliability and exclude unregulated activities. The particularly problematic conceptualization of the slum/informal settlements indicator is examined in depth, along with indicators on transport and waste management