Nature-based solutions, through the protection, restoration, management, and creation of new and novel urban ecosystems and the provision of ecosystem services, constitute a promising option for pursuing urban sustainability. Despite the scientific evidence of numerous environmental, social, and economic benefits, pilot projects remain the dominant implementation mechanism of NbS in urban contexts. Considering this, the EU policy and scientific community promote scaling NbS, that is, their systematic integration in urban planning and other decision-making processes to impact more people over a longer time frame. This thesis addresses three main scaling mechanisms that can contribute to mainstream NbS in urban areas: the application of NbS in multiple contexts (scaling out), changes in planning regime (scaling up), and new thinking and values (scaling deep). The first part of the thesis assessed the three scaling mechanisms through a qualitative content analysis of policy and planning documents in a real-life Spanish multilevel planning and governance context. Several scaling patterns were identified, upon which planning characteristics that hinder signs of progress on the scaling out, scaling up, and scaling deep of NbS, as well as opportunities, emerged. The rest of the thesis focused on the key challenges of the scaling-out mechanism. The implementation of multiple NbS was simulated stepwise for the case study of Las Palmas de Gran Canaria, and curves of cumulative impacts were quantified in terms of ES supply and beneficiaries. The non-linear relationships between NbS implementation and impacts served to discuss critical issues for planning NbS scaling-out strategies. The last study of the thesis investigated the design of specific NbS at the local scale for urban stormwater management, considered a determinant issue for ensuring the adaptability and efficiency of scaled-out NbS. A review of the scientific literature identified a broad set of design variables, related impacts on runoff mitigation and stormwater treatment, and assessment indicators. By critically analyzing the relationship between design variables and impacts, the thesis drew some NbS design recommendations for practitioners. The thesis concluded by providing several insights for the NbS planning and design that can facilitate pursuing scaling goals in urban areas. Finally, further research opportunities emerged concerning assessment methods in various urban contexts and how actions across governance levels and sectors, the role of actors’ coalitions, and co-production/co-learning of knowledge can aid in supporting the flourishment of NbS in cities.