This chapter centers on theory, research and instructional practices related to input and input processing. It is divided into five main sections. The first addresses how input, or samples of a target language to which a learner is exposed during communication, can be broken down in terms of different modalities (spoken, written and others) and different levels of linguistic analysis (phonology, lexis, morphosyntax and others). The second section underscores the role of input and input processing in the development of form-meaning mapping during language acquisition. The third offers a critical analysis of several models and theories related to input processing. The fourth section then focuses on language instruction, with particular attention to how the acquisition of grammar and vocabulary may be facilitated by instructional approaches grounded in research on input processing. This section, which highlights the use of structured input and input-based incremental vocabulary instruction, also proposes a graded taxonomy for classifying different techniques of input enhancement. The fifth section concludes by delineating current limitations on research in this area and spotlighting ways in which future research can help us to advance our understanding with regard to theory and to increase our options when it comes to evidence-based language instruction.