There is no agreement as to where the limits of consciousness lie, or even if these putative borders exist. Problems inherent to the study of consciousness continue to confound efforts to establish a universally accepted theory of consciousness. Consequently, clinical definitions of consciousness and unconsciousness are unavoidably arbitrary. Recently, a condition of severely altered consciousness has been described, which characterizes the borderzone between the vegetative state and so-called "normal" consciousness. This condition, referred to as the minimally conscious state (MCS), is distinguished from the vegetative state by the presence of minimal but clearly discernible behavioral evidence of self or environmental awareness. This chapter reviews the diagnostic criteria, pathophysiology, prognostic relevance, neurobehavioral assessment procedures and treatment implications associated with MCS.