The bispectral index (BIS) is processed electroencephalographic technology used in concert with clinical evaluations to objectively evaluate a patient's level of consciousness and probability of recall. Although the BIS has been extensively studied in the operating room setting, differences in patient populations, goals of treatment, and the environments themselves necessitate the development of BIS data specific to the intensive care unit. Data have evolved over the last several years, but for many reasons, the results and conclusions have varied. Yet within the data are important consistencies that help define the usefulness of BIS in patients who cannot be evaluated with subjective assessment tools such as the Sedation-Agitation Scale or the Richmond Agitation-Sedation Scale. Some of these patients cannot be evaluated with such tools because they lack motor responsiveness due to therapeutic paralysis or because they are receiving deep sedation. Bispectral index scores that are higher than expected in clinically sedated patients can often be traced to electromyographic activity or to the possibility of inadequate sedation and analgesia. The BIS must not be regarded as the sole indicator of level of consciousness, but should be used as part of an integrated approach to the evaluation of carefully selected patients with critical illness.