Many patients in the ICU receive mechanical ventilation and require sedative medications. Anxiolysis, hypnosis, and amnesia can be considered the primary objects of sedative therapy. Intravenous benzodiazepines are the drugs most commonly used for sedation in ICU. Proper choice and use of benzodiazepines is based on knowledge of the pharmacology and is an essential component of caring for patients in the intensive care unit. Three benzodiazepines -- Diazepam, Lorazepam and Midazolam -- are currently available for parenteral use in the ICU. Onset and duration of action are determined by their lipid solubility. Respiratory depression and hypotension are dose-dependent. Midazolam is generally preferred to other benzodiazepines in most ICU. It has the shortest half-life of the benzodiazepines, does not have active metabolites, is water soluble and can be administered by continuous infusion. Despite the relatively short half-life of Midazolam, extensive distribution can cause prolonged sedation. Recovery time is proportional to the infusions duration. Lorazepam is lipid soluble and dissolved in a propylene glycol carrier, produces a delayed onset and prolonged duration of effect and is preferred for long-term sedation ( > 48 hours). Propylene glycol toxicity is possible with high-dose or prolonged infusions. Diazepam has become less used with the introduction of the shorter-acting and less irritating benzodiazepine. The recent literature focuses on the differences between Midazolam and Propofol, the most used sedatives in ICU, their sequential use and combination. Relevant studies have been performed about propylene glycol toxicity.