The development of the concept of delirium spans nearly 2,500 years. Its core clinical features were recognized at least as early as the 16th century, while its management reflected a humane approach from the beginning of the modern era. In the 19th century delirium became linked with the concepts of disordered consciousness and confusion, but these two terms were also used in regard to certain functional mental disorders. The most important contribution in this century was the work of Engel, Romano, and associates, who postulated that the syndrome was due to reduction in brain metabolic rate, as reflected in slowing of the EEG background activity. These and other developments are discussed in this article. A list of proposed research priorities is included.