Open-vent volcanoes are special systems where the dynamics of sustained magmatic processes can be thoroughly investigated and where new monitoring tools can be tested and applied. However, various aspects remain puzzling at open-vent volcanoes for which forecasting their behaviour can be an important challenge. Recent papers highlight the very rapid improvements in spaceborn instruments, data acquisition techniques, data treatment and modelling over the last decade, and illustrate the fundamental contribution of long time-series data, either discontinuous or continuous, and the development of multi-parameter studies.Here we provide an interpretative overview of the main characteristics of open-vent volcanoes on the basis of selected examples chosen to be representative of the diversity of their magma composition, their eruptive activity and their geodynamic context. We choose typical open-vent volcanoes (Stromboli, Yasur and Erebus), some of them hosting a lava lake (Erta 'Ale, Nyiragongo, Villarrica, Ambrym and Masaya), to those with vigourous activity, which are associated to a longlasting eruption (Arenal, Fuego, Popocatépetl, Santiaguito). We briefly review their surface activity and report the values of SO 2 flux and the derived magma supply rate with emphasis on the key results found on their behaviour. We show the key role of the magma viscosity and its implication on the degassing. We present the current models used to explain how an open-vent volcano could be maintained, such as by the simultaneous rise and fall of a degassing and degassed magma (bidirectional flow models) and the few thermal models at lava lakes and in the conduit. Finally we discuss the sulphur evolution for three nearby volcanoes at the triple junction in Central America (Pacaya, Fuego, Santiaguito).
