In spite of lively criticism and debate, Krashen's ideas were very influential during a period when second language teaching was in transition from approaches that emphasized learning rules or memorizing dialogues to approaches that emphasized using language with a focus on meaning. Since then, COMMUNICATIVE LANGUAGE TEACHING, including IMMERSION and CONTENT-BASED INSTRUCTION, has been widely implemented, and Krashen's ideas have been a source of ideas for research in second language acquistion. Classroom research has confirmed that students can make a great deal of progress through exposure to comprehensible input without direct instruction. Studies have also shown, however, that students may reach a point from which they fail to make further progress on some features of the second language unless they also have access to guided instruction ( see Chapter 6). Some insights from learning theories developed in psychology help to explain why this may be so.