This chapter introduces the book's central field of inquiry by placing the psychology of musical development at the core of music education. It asks how knowledge about musical ability, experience, and age-related change can be translated into dependable pedagogical guidance rather than remaining a merely theoretical resource. The chapter identifies two persistent difficulties: the distance between the teacher and the contemporary child's musical world, and the extent to which educational choices depend on broader assumptions about development, culture, and environment. It also maps the logic of the chapters that follow and clarifies why research on early perception, enculturation, competence, and emotion matters directly for teachers, psychologists, and researchers. The introduction thus defines the book's purpose as a meeting point between empirical knowledge and responsible pedagogical judgment.