Adequate monitoring of analgesia and sedation should be one of the main goals in the Intensive Care Units. Once the analgesia is assured, the correct dosage of the sedatives will depend on the correct monitoring of the sedation level. For years, clinical scales have been used to evaluate the sedation level. These scales are appropriate for the evaluation of mild sedation but they cannot correctly evaluate deeply sedated patients or those patients who require continuous intravenous administration of neuromuscular blockade agents. In this chapter, we review the most used monitor to control the adequate level of sedation in the Intensive Care Units as well as the BIS(R) Monitor, which is the one recommended by the Analgesia and Sedation Work Group of the Spanish Society of Critical Care Medicine (SEMICYUC).