How people learn a second language (L2) and how we can teach one and assess student learning most effectively have always been subjects of great interest. As curiosity is the mother of all invention, theory generation and experimentation are natural responses to all mysteries. Although in the field of L2 education, many theories and educational models have been presented, some have enjoyed more attention in research and investment than others. Thus far, previous books on L2 learning, teaching and assessment have exhibited three common approaches in their content foci. Whether as an introduction to the field (e.g. Atkinson, 2011; Gass & Selinker, 2008), or on a more specialized topic (e.g. Cook & Singleton, 2014), one approach has focused only on second language acquisition (L2A). The second approach has concentrated only on methods and techniques in L2 teaching (e.g. Richards & Rodgers, 2010), while a third one has taken a more blended style, introducing L2A and teaching together (e.g. Cook, 2008; Horwitz, 2013).

In line with its theoretical and pedagogical view and intended audience, each approach has highlighted different aspects and elements of five key components of L2A, pedagogy and assessment: the learner, teacher, environment, interaction and input. Topics of study have included particular groups of L2A theories (e.g. sociocultural theory), certain learner characteristics (e.g. motivation), some teacher-related factors (e.g. teacher cognition: Borg, 2006), special instructional methods (e.g. communicative language teaching) and certain testing approaches, among others. In doing so, some placed the ‘L2 learner’ (Dörnyei et al., 2015) or certain ‘testing methods’ (Hughes, 2003) at the center of the discussion, while others used ‘conversational interactions’ (Mackey, 2007) or a ‘skill-based instructional method’ (Larsen-Freeman, 2003) as a basis.

What is missing in the field, however, is an exploration of these subjects from the perspective of the role of comprehensible input (CI), or input and its comprehension. Such a need is warranted because the study of the learner, teacher, environment and interaction factors in L2A and pedagogy cannot be separated from that of input.