Summary Sedation monitoring in intensive care unit (ICU) is necessary to avoid the problems related to both under and over sedation but there is no gold standard. The aim of this article is to review the methods used for sedation monitoring in ICU. Each method should be evaluated in terms of validity, applicability, responsivity, intra-rater reliability and inter-rater reliability. Both subjective and objective methods for monitoring sedation are available. Subjective methods include clinical scales; the Ramsay Scale, the Glasgow Coma Scale modified by Cook and Palma, the Sedation Agitation Scale, the Richmond Agitation and Sedation Scale, the Adaptation to the Intensive Care Environment (ATICE) scale and the Comfort scale for paediatric patients. Objective methods include: pharmacokinetic methods, lower oesophageal sphincter contractility measurement, heart rate variability measurement and neurophysiologic methods. Recently technological developments use neurophysiology and these include evoked potentials, electroencephalography and derived parameters such as Bispectral Index, Entropy, Patient State Index and Narcotrend. When neuromuscular blockade or sedation is administered to patients objective methods are fundamental to assessment of the sedation level. Studies of objective methods produce controversial results suggesting the ideal method is not yet available.