The "positive youth development" (PYD) perspective is an orientation to young people that has arisen because of interest among developmental scientists in using developmental systems, or dynamic, models of human behaviour and development for understanding (1) the plasticity of human development and (2) the importance of relations between individuals and their real-world ecological settings as bases of variation in the course of human development (Lerner, 2005). The PYD perspective has arisen as well through the development and, in some cases, the evaluation of interventions designed and delivered within community-based, youth serving programs that have worked to counter what have been seen as steady states across the past five to six decades of substantial incidences of risk behaviours among adolescents. This book discusses several of the key models of PYD framing the literature of developmental science. In addition, we illustrate the use of the PYD perspective in understanding adolescent development in relation to the multiple contexts of youth development and in promoting PYD through community-based interventions or social policy. In turn, this chapter rationalizes and explains the foci of this book by discussing the origins and the features of the PYD perspective.