This part aims to describe the role of comprehensible input (CI) in second language acquisition (L2A) theory and research in four chapters. It particularly focuses on cognitive processing oriented, learner-centered and socially oriented views of L2A. The section on cognition-based L2A theories covers topics pertaining to the role of CI in cognition and L2 learning processing, first language-based inferences, information processing, attention (focal/peripheral), control (controlled/automatic) and knowledge processes (declarative/procedural and explicit/implicit). Schema theory and connectionism are particularly highlighted here. Next are brief sections on the intersection of CI and the nature of processes of L2 attainment (acquisition/learning and monitor model), the nature of input (natural order, CI and i+1), the nature of processing and production (innate acquisition resources and silent period) and the role of affective factors (e.g. motivation) in L2A. Here, there is also a brief section on CI’s role in the Interaction and Output Hypotheses. The section on the role of CI in socially oriented L2A theories and research covers the following topics: social distance, psychological distance, outcomes of social and psychological distance, individual versus society in L2A, language socialization and activity theory. This part ends with a brief section on the use of CI in socio-cognitive, dynamic systems, ecological, complexity/chaos, critical period and interlanguage theories of L2A.