General anesthesia has three partially independent components: hypnosis, paralysis, and analgesia.During general anesthesia, the conscious experience of pain disappears due to hypnosis, and surgical stimulus with its immediate consequences on system function is recognized as nociception.As contrast to depth of hypnosis (e.g.with bispectral index or spectral entropy) or level of paralysis (e.g.train-of-four) monitoring, which are monotonically dependent on the corresponding drug effects, the depth of analgesia monitoring is dependent on the balance between the input (noxious stimulus) and the output (the nociceptive response).Autonomic reactions, such as tachycardia, hypertension, sweating and lacrimation, although non-specific, are always regarded as signs of nociception or inadequate analgesia.A more reliable quantification of the nociceptive-antinociceptive balance has been attempted by analyzing reflex pathways, skin vasomotor reflexes, pulse plethysmographic signal, pupillometry, and heart rate variability.More recently, a new variable called Analgesia/Nociception Index (ANI) derived from heart rate variability and reflecting the parasympathetic tone was proposed.The ANI monitor was developed by CIC-IT research unit of Lille, France and is now commercially available (MDoloris Medical Systems, Loos, France) in many countries throughout the world.The purpose of this presentation is to describe the calculation of ANI and to present the results of clinical studies performed in awake and anesthetized patients.