Although second language acquisition (SLA) and language teaching are two distinct fields, an increasing number of classroom-based SLA research on the effectiveness of different types of instruction, such as focus on forms, focus on meaning, and focus on form (see Norris & Ortega, 2000 for a review), corrective feedback (e.g., Lyster & Ranta, 1997), input enhancement (e.g., White, 1998), etc., has brought about a burgeoning subfield of SLA known as Instructed SLA (ISLA). ISLA, which is often pitted against naturalistic SLA, "occurs in formal settings where language is intentionally taught and intentionally learned--within a limited amount of time" (Spada, 2015, p. 71). Given ISLA's immediate and potential relevance to second language (L2) pedagogy, Long (2014) made the first attempt to formulate a cognitive-interactionist theory of ISLA. In what follows, this theory will first be presented and then discussed in relation to its significance to and implications for task-based language teaching (TBLT) and syllabus design.