This chapter describes positive youth development (PYD) as an emerging arena of applied developmental science. We show how PYD is both rooted in the theoretical traditions of developmental psychology, and fueled by newer emphases on nurturing the potentialities of youth more than addressing their supposed deficits, and on addressing and helping to shape the roles of developmental contexts, especially that of the community, and youth themselves as agents of their own development. We begin with an historical overview and a presentation on major conceptual frameworks, including the framework of developmental assets, which have significantly influenced PYD theory, research, and programs. The following section of the chapter poses seven broad hypotheses that represent the scientific foundation of PYD (e.g., contexts can be intentionally altered to enhance developmental success and changes in the context change the person), and reviews a wide array of studies that lends support to the hypotheses. After demonstrating the general utility of these hypotheses for understanding and promoting positive development in all youth, we review research that illustrates differences in developmental paths and outcomes across youth diversity in gender, age, race/ethnicity, and socioeconomic status. The chapter ends by suggesting those areas of youth development knowledge where the field is relatively strong (e.g., taxonomies of factors that are correlated with positive outcomes), and those areas where significantly more research is needed (e.g., theories of change that articulate how youth, adults, and community systems move toward greater developmental attentiveness). We conclude by posing a number of theoretical questions, research challenges, and applied needs to be addressed if the promise of PYD as both a scientific and applied field is to be fully realized.