Cities pose environmental challenges but also offer possibilities to close material and energy loops and connect multiple societal and ecologic services. This article reviews and brings together the literature on two important new research directions that address urban sustainability by integrating functions or material flows: Circular Urban Metabolism (CUM) and Multifunctional Land Use (MLU). We focus on challenges to MLU and CUM and strategies to facilitate their realization. The review shows that although MLU and CUM differ in what they integrate, they face partly similar integration challenges. In both fields, the collaboration between actors related to particular functions (water safety, recreation), high investment costs and uncertainties about costs and benefits, and legislation that hampers integration are identified as challenges. In both fields, strategies are proposed to facilitate the collaboration between actors. However, other challenges and strategies are specific. Whilst MLU scholars mostly highlight socio-economic aspects of realizing integration, CUM scholars focus more on technical aspects. We find limited cross-fertilization between both fields so far. To stimulate discussion and knowledge exchange, we introduce ‘integration of urban functions’ as a shared idea for a sustainable urban system. To find further solutions for integration challenges, we propose conceptualizing MLU or CUM initiatives as processes of change, which requires connecting across previously separate ‘worlds’ and changing previously established monofunctional ways of working.