While other volcanic gas species are more abundant than sulfur, it is the measurement of sulfur dioxide emissions that has played the key geochemical role in volcano monitoring for decades. Recently, this sphere of volcano surveillance has undergone a revolution: the instruments suitable for the task have become cheaper, more compact, less power hungry, and more capable than their predecessors. It is now possible to measure multiple gas species simultaneously, at high time resolution, and even to image volcanic clouds remotely. This technological explosion is leading to the installation of a global network of volcanic-emission sensors. This network will underpin the geochemical surveillance and hazard assessment at volcano observatories worldwide and will yield new insights into the degassing and eruptive style of volcanoes and the impact of volcanic clouds on the atmosphere.