Defining the extent of unconscious processing is an important step for understanding the specificity of consciousness. In this talk, I will focus on whether there are qualitative limits to unconscious perception. I will present several behavioral and brain imaging studies (EEG, fMRI) aimed at characterizing the depth of processing associated with unconscious stimuli. I will present the patterns of neural activity associated with subliminal priming in several domains (written words, faces, speech) and how they differ from those induced by visible stimuli. I will then focus on the cerebral distinction between subliminal perception and conscious access, and on the special status of perception without attention. I will conclude on the complex issues of measuring consciousness in the absence of report and/or attention.