This chapter traces post–World War II transformations in the study of happiness, from humanistic psychology to contemporary policy debates. It first examines foundations laid by figures such as Viktor Frankl and Abraham Maslow, highlighting the shift from pathology to growth, self-actualization, and meaning as central to well-being. It then turns to the “beyond GDP” movement and the Easterlin paradox, which challenge the assumption that rising income reliably increases happiness and motivate broader quality-of-life indicators. The chapter next surveys the rise of positive psychology, its institutionalization, and the development of positive psychology interventions, alongside substantial critiques concerning theory, methods, neoliberal individualism, and commercialization. A final section reviews proposed “waves” and “bands” in happiness science, the growing integration of hedonic and eudaimonic perspectives, and the expansion toward systems, cultural context, and global metrics such as the World Happiness Report. Overall, postwar happiness research emerges as a dynamic, contested field spanning individual experience, social structures, and national and international policy agendas.