Recent decades have seen a considerable renaissance of scienti?c interest in the study of human consciousness. For the purposes of descriptive clinical psychopathology, consciousness can be simply de?ned as a state of awareness of the self and the environment. Disorders of consciousness are associated with disorders of perception, attention, attitudes, thinking, registration, and orientation. Consciousness can be changed in three basic ways: it may be dream-like, depressed, or restricted. This chapter outlines these different types of disturbance of consciousness, including delirium, twilight states, and dissociative fugue, among other conditions. The chapter concludes with suggested questions for eliciting specific symptoms in clinical practice, in addition to standard history-taking and mental state examination.