This chapter describes the rationale and conceptual foundations for the Promoting Psychological Well-Being Globally (PPWBG) project that is the focus of this book. The purpose of the multi-country project was to explore psychological well-being of children and adolescents from the perspectives of multiple stakeholders (children, adolescents, parents, teachers, school administrators, and support staff). Data were collected by research teams led by local school/educational psychologists in 14 sites across 12 countries: Brazil, Estonia, Greece, India, Italy, Mexico, Romania, Russia, Slovakia, Sri Lanka, Tanzania, and the USA. The conceptual foundations for the project included Bronfenbrenner’s Ecological Systems Theory, a conceptual model of psychological well-being that included individual and cultural factors, and a child rights framework. The research process was guided by the participatory culture-specific intervention model, with the goal of formulating culturally and contextually relevant definitions of psychological well-being that could inform development of universal (shared) and local (site-specific) models for promoting well-being of children and adolescents.