Against the backdrop of rapid global urbanization—particularly within high-density Asian megacities—this chapter identifies the critical nexus between the built environment and residents’ subjective well-being (SWB), establishing it as a key academic and policy priority. It aims to dissect the complex, dualistic role of high-density urban settings in shaping SWB, within China’s unique context. The chapter begins by presenting the urban well-being paradox: despite unprecedented economic growth and urbanization, China’s SWB rankings lag, pointing to the limits of growth-focused models. Using an interdisciplinary approach, it reviews theoretical foundations, empirical evidence, and policy shifts toward people-centered urban development. Section 1.1 outlines the historical and philosophical underpinnings of SWB and its measurement, tracing the shift from endogenous to exogenous factors in well-being research. Section 1.2 analyzes the dualities of high-density urban living—balancing opportunities with challenges. Section 1.3 critiques current intuitive and Western-centric discourses, identifies research gaps (mediating pathways, contextual specificity), and declares the book’s mission to develop a mechanistic, context-sensitive framework linking built environment attributes to SWB via perceptual and social mediators. Section 1.4 operationalizes key concepts—SWB, built environment (using the “5D” model), and high-density cities—providing a foundation for subsequent empirical analysis. Integrating urban planning, psychology, and policy, this chapter provides a foundation to guide the design of high-density cities that enhance well-being and support sustainable urban development.