Functional neuroimaging, electroencephalography and clinical examination are essential to understand the many facets of disorders of consciousness. Yet, it is still not widely known that patients exist who are clearly conscious but unable to show it owing to complete loss of motor output, a condition which has been termed cognitive motor dissociation. In this review, the mechanisms of normal and impaired consciousness are discussed, and current theories of consciousness, as well as specific clinical signs, which are essential to discern the state of consciousness in a given patient, are highlighted.