Communicative Language Teaching or CLT is an approach to second language teaching which grew out of Krashen and Terrel's Natural Approach.  It is based on Krashen's Monitor Model, or input hypothesis, and also draws on the notion of communicative competence.

The Monitor Model focuses on five elements:

1. acquisition versus learning: explicit grammar instruction does not facilitate acquisition
2. natural order: teachers must respect learners' developmental sequences
3. comprehensible input (i + 1): the necessary and sufficient condition for acquisition
4. monitor: explicit grammar knowledge is only useful for checking writing
5. affective filter: learners must be relaxed and motivated in order to learn.

CLT came to prominence in the 1980s and remains popular in many language classrooms today.  It is favoured by learners who aim to understand and communicate in the contemporary language, perhaps in informal contexts. It also appeals to teachers, particularly less experienced native speakers who feel uncomfortable with the technical aspects of phonology and syntax, and teachers of younger learners who embrace constructivist principles. 

Many teachers and learners however reject Krashen's extreme position in banning explicit grammar instruction, and CLT is nowadays frequently combined with focus on form or other types of grammatical instruction.

Read more about direct versus indirect methods of second language teaching.