This chapter lays the theoretical foundation of the book by providing an up-to-date literature review of CBA. It begins with a discussion of the paradigm shift in the field of educational assessment, highlighting that CBA has gained prominence over the past decades. It then discusses the definitions and purposes of CBA, emphasizing that, influenced by the culture of assessment for learning, the overarching aim of CBA has been conceptualized as facilitating learning. Following this, the chapter discusses the principles of using CBA for learning purpose and introduces a framework for the practice of effective CBA. Then it reviews studies on CBA conducted in L2 educational contexts, discussing the current status of L2 teachers’ CBA implementation and various challenges faced by L2 teachers. A salient gap emerging from the literature is that limited attention has been paid to young language learners. The final part of this chapter thus focuses on CBA for young language learners, a learner group with unique characteristics. It discusses how assessment of YLLs has evolved as an independent area of inquiry and examines the benefits, strategies and current research related to CBA for YLLs.