Aim of this work is to provide an overview on the main clinical issues concerning disorder of consciousness (DOC). After a briefly description of the debate on clinical differences in states of altered consciousness, we report the description of clinical features of the three different levels of DOC: coma, vegetative state, and minimally conscious state, according to the Multi Society Task Force for Persistent Vegetative State (1994) and the Aspen Work Group (Giacino et al., 2002). We will then describe an observation procedure, stated by Whyte and coworkers in 1999, based upon a single-case methodology aimed to assess responsiveness and its variations. At least, we will give a description of the evidences on stimulation treatment efficacy, as we collected in occasion of the last Consensus Conference in Neuropsychological Rehabilitation held in Siena (Italy) in 2010. Our conclusions confirm the lack of evidences concerning the efficacy of treatment for recovery of consciousness in agreement with other authors and we will finally provide suggestions for future research