This article reviews the literature on "poor insight" or unawareness of illness in schizophrenia. A large body of knowledge representing several different perspectives on insight has developed. This work can be divided into three broad categories, suggesting an important role for insight in the phenomenology, pathophysiology, and treatment of schizophrenia. The argument is made here that many of the self-awareness deficits observed in schizophrenia are of diagnostic significance, are neurally based, and are indispensable in guiding treatment decisions. In addition, this article proposes guidelines for assessing unawareness of illness in schizophrenia and discusses the relevance of such deficits to the diagnosis of schizophrenia.