In this book, Krashen puts for his Input Hypothesis. The Input Hypothesis attempts to account for the processes in which languages are learned. According to Krashen, the only way in which learners can acquire language is through comprehensible input which is slightly above the learners level. One surprising point he makes is that formal learning never leads to acqusition. The primary method of getting comprehensible input is through listening or reading. In other words, it is not so important for learners to practice speaking to acquire language.
While some of Krashen's claims are interesting, in the end his hypothesis doesn't hold water. Several of his terms aren't well defined and his theories prove impossible to experiment with. The studies which he claims support his hypothesis either don't really support it or only support it indirectly. A final complaint is that some of the assumptions he makes rests on faulty concepts. For example, he misapplies the concept of Chomsky's Language Acquisition Device (LAD).
In the end, the input hypothesis proves to be less than promised. However, students of applied linguistics may find this book interesting as it illuminates some of the assumptions that we take for granted. In bringing these assumptions into question, this book provides something of benefit to the student of applied linguistics.