This chapter introduces the book’s conceptual, ethical, and analytical foundations by positioning the Capability Approach as a framework for social impact assessment (SIA) in development practice. Drawing on Amartya Sen’s conception of development as the expansion of substantive freedoms, it argues that conventional output- and indicator-driven evaluations are insufficient for assessing whether development interventions genuinely enhance people’s well-being, agency, and dignity—particularly for women, people with disabilities, and other marginalised groups. Rather than asking whether projects deliver predefined outputs, the book reframes SIA as an inquiry into what changes people themselves value, for whom these changes occur, and under what social and institutional conditions. The chapter sets out an analytical framework centred on three determinants of capability change: effective access to resources, effective agency, and socio-structural context. These concepts guide the analyses that follow, establishing a people-centred and ethically grounded approach to evaluating WASH (water, sanitation, and hygiene) development projects in Vietnam and Cambodia, with a focus on gender, disability, and social inclusion.