This open access book explores how to operationalise circular economy principles in the built environment by turning barriers into enablers of systemic change. Drawing on research from the COST Action CircularB, it examines technical, regulatory, economic, and cultural obstacles while highlighting the enabling role of digital tools, innovative business models, and harmonised indicators. The book examines how circularity is integrated into European and national policies, public procurement, and urban strategies, offering a comparative analysis of best practices and policy gaps. It also addresses the need for standardised terminology and frameworks to support the integration of circularity in design, assessment, and construction. Featuring a curated glossary of over 400 terms, the book fosters shared understanding across disciplines. Combining critical analysis with actionable insights, it equips professionals and policymakers with tools to support a regenerative and resource-efficient transition in the construction and urban sectors.