To guide the readers in progressively connecting theory, research and practice about the role of comprehensible input (CI) in second language acquisition (L2A), teaching and assessment, Part 1 (Chapters 1– 3) describes the theoretical foundation and conceptual bases of the concept of CI. Firstly, definitions of basic constructs such as comprehension, comprehensibility, input, intake, output and CI are provided. Here, input’s linguistic, cultural, semiotic and stylistic codes are particularly highlighted. Building on the cumulative wisdom in related fields, an alternative view of CI is also offered. Secondly, the conceptual underpinnings of CI are explicated from the perspectives of cognitive load, automaticity, schema, dual-coding, multimodality, constructivism, socio-constructivism, poststructuralism and hybridity theories. Finally, the constitutive elements of CI are described and exemplified from the viewpoints of five different fields of linguistics, including applied linguistics, sociolinguistics, psycholinguistics, neurolinguistics and computational linguistics.