The building sector contributes around 39% of global carbon dioxide emissions and consumes nearly 40% of all the energy produced. Over the whole life cycle, the building sector yields over 35% of the EU's total waste generation. These facts substantiate the necessity to implement circular economy in the built environments, in order to mitigate global warming and climate change emergency. This chapter highlights the state-of-the-art knowledge and research gap with respect to the stakeholders’ influences, inter-relationships, and obstacles for circular economy implementation on building stocks. In this chapter, a robust critical literature review of key documentations such as research articles, industry standards, policy reports, strategic roadmaps, case studies, and white papers has been rigorously conducted together with expert interviews. The state-of-the-art review addresses multi scales of CE practices adopted within the built environments. This chapter spells out current challenges and obstacles often encountered by various stakeholders. Case studies related to circular economy implementation have been drawn in order to promote such the CE practices across value chains in different regions and counties; and to overcome the barriers for circular economy implementation.