Background: Pain management for unconscious or sedated critical care adults in Intensive Care Units (ICUs) is based mainly on the use of a reliable, valid and feasible pain assessment tool. Aim: The comparative evaluation of the validity and the reliability of pain assessment tools appropriate for sedated or unconscious critical care adults, who are not able to communicate. Method: A literature review was conducted with the use of the electronic databases PubMed and CINAHL for articles published 1991−2011, using the key-words: “Pain”, “assess”, “measure”, “tool”, “instrument”, “intensive care”, “critical care”, “unconscious”, “sedation”, “adult” and “validity”. The search was made for original English or Greek language articles presenting the development of a pain assessment tool appropriate for sedated or unconscious critical care adults, or measuring the validity and reliability of such tools. Results: The review included 13 studies, of which 6 articles introduced a new pain assessment tool and 7 validated such tools. All the pain assessment scales included behavioural parameters, 3 included physiological categories and 2 assessed compliance with mechanical ventilation. The evaluation of the scales was made through assessment of internal consistency, inter-rater reliability, criterion validity and discriminant validity. The highest internal consistency was found for the Critical- Care Pain Observation Tool (CPOT), while the highest inter-rater reliability was for the Behavioural Pain Scale (BPS). These two scales, to date, have been found to be the most valid and reliable, although with only small advantage over the others. Conclusions: Pain assessment scales for critical care adults are fairly new tools and therefore their reliability and validity has not yet been evaluated sufficiently. Further studies are needed in order to establish a “gold standard” tool for critical care patients.