This chapter provides a general introduction to the topic of this book: namely, the conceptualization of assessment for learning (AfL) and the challenges of its implementation. It addresses theoretical issues, including the definition of assessment for learning and its relations with other concepts—in particular, the formative and summative functions of assessment. It discusses the characteristics of student learning to be considered in designing AfL, as well as the external constraints and other practical considerations that influence the implementation of AfL. In conclusion, it presents the structure of the book in three parts dealing with three interrelated aspects of AfL implementation: policy, professional development, and classroom practice.