Pain is a common and distressing symptom in ICU patients. Yet a major challenge exists in assessing and evaluating the pain. Although the patient's self-report of pain is the “gold standard” for pain assessment, other methods must be considered when patients are unable to self-report. Currently only two pain behavior instruments have been tested for their reliability, validity, and feasibility of use in ICUs: the pain behavior scale and the Critical-Care Pain Observation Tool. Other tools, albeit with less validity testing, include the pain assessment, intervention, and notation (PAIN) algorithm and a pain behaviors checklist. When established tools are insufficient to evaluate a patient's pain, alternative methods of augmenting a pain evaluation should be considered. These can include the completion of a pain risk profile, use of surrogates, or performance of an analgesic trial. Meticulous attention to the evaluation of a critically ill patient's pain provides the basis for selection of pain interventions and the subsequent assessment of the intervention's effectiveness.