The circular economy represents a paradigm shift in production and consumption, aiming to minimize waste, maximize resource efficiency, and extend product lifespans. This chapter explores the foundations, principles, and sustainable design strategies that underpin circular systems. Traditional linear models based on a take–make–dispose approach are increasingly recognized as unsustainable due to environmental degradation, resource depletion, and socio-economic pressures. In contrast, the circular economy emphasizes designing out waste, keeping materials in use, regenerating natural systems, and retaining value throughout the lifecycle. The chapter highlights the strategic role of sustainable and circular design, including durability, modularity, repairability, and upgradability, as essential enablers for achieving longevity and functional adaptability in products. Lifecycle thinking, systems-oriented approaches, and user-centred design are presented as critical tools for integrating environmental, economic, and social dimensions. Tools and frameworks, such as life cycle assessment (LCA), material flow analysis (MFA), digital twins, eco-design platforms, and circularity indicators, provide actionable insights to implement circular strategies effectively. Policy, governance, and institutional mechanisms, including extended producer responsibility, eco-design directives, certifications, and public–private collaboration, are discussed as drivers for systemic adoption. The chapter also examines barriers, such as technical limitations, market constraints, organizational resistance, and knowledge gaps, while outlining emerging directions, including digitalization, regenerative approaches, capacity building, and innovation pathways.