As the world embraces sustainable practices, the concept of circularity has become increasingly important, especially in the context of existing buildings. This chapter comprehensively analyses circularity tools and methods, focusing on their application in the built environment. By examining a variety of tools—both quantitative and qualitative—we explore their methodologies, information requirements, and levels of detail. This structured approach systematically evaluates products, materials, and systems in terms of their potential to support a closed-loop materials flow. Our analysis highlights the necessity of continuously updating and refining these tools to incorporate the latest data, trends, and technologies, guiding the construction and urban development sectors toward a more sustainable future. The chapter is a valuable resource for practitioners, researchers, and policymakers seeking to enhance the sustainability of existing buildings.