The interpretation of the EEG in the critically ill remains a clinical challenge. Because continuous EEG monitoring plays an increasing role in patients' care, it is important that research efforts investigate the clinical significance of periodic and rhythmic discharges and of background abnormalities. The 2012 American Clinical Neurophysiology Society critical care EEG terminology was designed to provide a comprehensive and objective vocabulary for that purpose. The interrater reliability of most of the proposed terms has been established, confirming that they represent a solid basis for research. Studies using the terminology have already started to define the clinical and prognostic values of several known or newly described EEG patterns. Yet, as the field of critical care EEG evolves, improvements will be required to further enhance the clarity of the terminology and incorporate new findings from ongoing research.