Background: Enacting good science in pursuing the international study of positive youth development (PYD) requires using sound developmental theory to formulate questions that are tested through employing rigorous, change-sensitive measures, designs, and data analyses that are aimed at describing, explaining, or optimizing thriving among the diverse youth of the majority world. Objective: We discuss the usefulness of theoretical models derived from relational developmental systems metatheory in framing such science, and we describe innovations in methodology that enable the specific pathways of development of majority-world youth to be understood and enhanced. Methods: Literature review and theoretical commentary. Results: We make recommendations for creating progress in the ways that the international study of PYD may contribute to policies and programs promoting lives of personal thriving and social contributions among the diverse youth of our world. Conclusions: Advances in the international study of PYD rest upon the use of non-reductionist, dynamic, relational theories of human development in the conceptualization of research and upon the use of change-sensitive measures, research designs, and data analysis procedures in the studies derived from such theoretical conceptualizations.