The Monitor Model is Stephen Krashen's unified theory of second language acquisition, developed across the late 1970s and 1980s. It comprises five interrelated hypotheses that together make a strong claim: acquisition is driven by comprehensible input, and conscious learning plays only a marginal editing role. Despite extensive criticism, the model remains one of the most widely known and debated frameworks in SLA, and its core ideas have profoundly shaped communicative and input-based approaches to language teaching.
There are two independent ways of developing knowledge of a second language:
Krashen's central claim is that these systems are entirely separate: acquired knowledge drives fluent production, while learned knowledge can only function as a monitor (see below). This strict non-interface position is the model's most controversial aspect.
Consciously learned knowledge serves only as a monitor, an editor that checks and corrects output after it has been initiated by the acquired system. The monitor can operate only when three conditions are met simultaneously:
In spontaneous conversation, these conditions are rarely all present, which is why Krashen argues that learning contributes little to real-time communication. He identified three learner types: Monitor over-users (hesitant, self-conscious), Monitor under-users (fluent but error-prone), and optimal Monitor users (who deploy conscious knowledge appropriately).
Grammatical structures are acquired in a predictable order, and this order is largely independent of the order in which structures are taught. Drawing on morpheme studies by Dulay and Burt (1974) and others, Krashen argued that the acquisition sequence is driven by internal processing mechanisms, not instructional sequencing. This does not mean grammar teaching is useless, but that it cannot override natural developmental sequences.
Acquisition occurs when learners receive Comprehensible Input at a level slightly beyond their current competence, expressed as i+1 (where i = current level). The learner uses context, world knowledge, and extralinguistic information to make sense of input containing unfamiliar structures, and acquisition happens as a byproduct of understanding messages.
Key claims:
This is the hypothesis that has had the greatest practical impact, influencing extensive reading/listening programmes, sheltered instruction, and content-based teaching.
Emotional variables such as motivation, self-confidence, and anxiety act as a filter that can block input from reaching the language acquisition device. When the affective filter is high (the learner is anxious, unmotivated, or self-conscious), even comprehensible input will not be acquired. A low affective filter creates conditions favourable for acquisition.
The Monitor Model has been challenged on multiple fronts:
Despite its flaws, the Monitor Model shifted the field's centre of gravity from teaching to acquisition, from form to meaning, and from output practice to input provision. Krashen's advocacy for free voluntary reading, comprehensible input, and low-anxiety classrooms has had lasting practical impact. The model is best understood not as a complete theory but as a set of powerful hypotheses that provoked more rigorous research, including the Interaction Hypothesis, Output Hypothesis, and Noticing Hypothesis, all developed partly in response to its limitations.
The Monitor Model connects to the Input Hypothesis and Comprehensible Input as its most influential component, to the Noticing Hypothesis and Focus on Form as direct challenges to its non-interface position, to the Interaction Hypothesis and Output Hypothesis as alternative accounts of what drives acquisition, and to Accuracy and Fluency as the tension the Monitor Hypothesis attempts to explain.