European cities are accelerating digital transformation through Urban Data Platforms that integrate data to guide policy, operations, and services. The Scalable Cities expert report on Urban Data Platforms provides actionable guidance to develop, implement, and scale open platforms, based on desk research covering 128 cases across Europe. It addresses the gap between the high potential of platforms and their limited current adoption, and targets city leaders, the SCC community, researchers, industry, and civil society. The work aligns with EU priorities for digital transformation, the Green Deal, and the Sustainable Development Goals. Deliverables include a state of the art overview, case studies of governance and technology, a comparative analysis of enablers and barriers, and clear guidelines and policy recommendations, plus insights on emerging trends. The aim is to help cities accelerate urban digital transformation and unlock durable public value.