The nociception-antinociception (NAN) balance is the ratio between the intensity of the nociceptive stimulus and the efficiency of antinociceptive treatments. Four NAN monitors have been mainly studied in the last decade and have been developed to optimise the administration of analgesics, particularly opioids, during general anaesthesia. Three of them, which may be called “nociception monitors”, provide indexes increasing with sympathetic activity related to nociception: the automated pupillometry (Algiscan®, ID-MED, Marseilles, France), the Surgical Pleth Index® or SPI (GE Healthcare, Helsinki, Finland) and the Nociception Level® (NoL) monitor (Medasense, Ramat Gan, Israel). In contrast, the ANI monitor® (MDoloris Medical Systems, Lille, France) provides an index decreasing in case of sympathetic activity related to nociception and increasing with parasympathetic activity related to antinociception, the Analgesia/Nociception Index (ANI), which explains why this monitor may be called an “analgesia monitor”. Another monitor provides a beat-to-beat index derived from the inhibition of the baroreflex by the autonomic nervous system (ANS) related to increased sympathetic activity during nociception, the Cardiovascular Depth of Analgesia® (CARDEAN) index (CARDEAN 4.0® Alpha-2, Lyon, France). A few monitors also provide indexes derived from the electroencephalogram (EEG), such as the Spectral Entropy (GE Healthcare, Helsinki, Finland) or the CONOX® (Fresenius-Kabi, Brézins, France), displaying two indexes, the qCON related to the level of hypnosis and the qNOX, associated with the level of nociception. In this chapter, we will try to categorise these different monitors based on their different approaches to estimating the NAN balance. Apart from theoretical aspects and calculation principles, we will also present an overview of clinical results with these different monitors and discuss their limits of use.