Nutrition research, ranging from molecular to population levels and all points along this spectrum, is exploring new frontiers as new technologies and societal changes create new possibilities and demands. This paper defines a set of frontiers at the population level that are being created by the increased societal recognition of the importance of nutrition; its connection to urgent health, social, and environmental problems; and the need for effective and sustainable solutions at the population level. The frontiers are defined in terms of why, what, who, and how we study at the population level and the disciplinary foundations for that research. The paper provides illustrations of research along some of these frontiers, an overarching framework for population nutrition research, and access to some of the literature from outside of nutrition that can enhance the intellectual coherence, practical utility, and societal benefit of population nutrition research. The frontiers defined in this paper build on earlier forward-looking efforts by the American Society for Nutrition and extend these efforts in significant ways. The American Society for Nutrition and its members can play pivotal roles in advancing these frontiers by addressing a number of well-recognized challenges associated with transdisciplinary and engaged research. Adv. Nutr. 4: 92-114, 2013.