The development of Krashen`s theory of second language acquisition has been disadvantaged due to the dispute over its untestable hypotheses. In his Monitor Model Krashen (1981; 1982; 1985) claims that linguistic competence can only be acquired subconsciously, while conscious learning mainly depends on learners’ mood and emotions at the time of learning a second language. A review of the studies investigating the coherence of Krashen’s hypotheses derived from the Monitor Model supported the validity of this approach. However, competing theories emphasize the importance of conscious learning not addressed by the Monitor Model. This paper critically reviews the five key aspects of Krashen’s Monitor Model and closely looks at the relevance of the theory and its characteristics to SLA nowadays. It was concluded that while effective in some classroom applications, the Monitor Model is too restrictive to justify the conscious learning as a source of spontaneous language production, so cannot alone provide a comprehensive account of language competence.