This book examines why urban planning continues to fall short of its aspirations for equity, sustainability, and resilience. It argues that contemporary challenges cannot be met without fundamentally reconfiguring how knowledge, power, and action are organized. The book present de-siloing as the essential groundwork for closing the persistent gap between planning ideals and lived urban realities. Going beyond procedural calls for coordination or collaboration, the book reframes de-siloing as an epistemic and political project. It maps the “ecosystem of siloing” across governance, academic, professional, and civic domains, showing how these structures shape what counts as legitimate knowledge and whose voices guide urban change. Drawing on global theory, practice, and case studies across diverse geographies, the book offers a framework for understanding and operationalizing institutional transformation—highlighting pathways that embrace plural knowledge, foster collaboration, and support more relational forms of planning. Bringing together scholars, practitioners, and activists, the book invites a wide audience to rethink planning not as the work of isolated experts but as a collective, situated practice. It is written for planners, policymakers, researchers, and students seeking more just, context-responsive, and interconnected urban futures.