Monitoring gases from andesite volcanoes for hazard mitigation or scientific enquiry is complicated by the wide range of eruption styles. Monitoring is aimed at both measuring the rates of gas emission, and changes in their compositions. Direct sampling techniques are restricted to accessible vents, and are unsuitable for syn-eruption monitoring. Correlation spectroscopy is a simple and robust method for measuring emission rates of sulphur dioxide, but is subject to large errors. Open-path Fourier transform spectroscopy provides a remote method for determining plume gas compositions, but requires careful atmospheric radiative transfer modelling. Few andesite volcanoes have been consistently monitored. Published data show that there is no simple general model for volcano degassing: each volcano, and each eruption, presents separate problems, many of them arising from the evolving interaction between magmatic and hydrothermal systems during an episode of activity. Because of its lower solubility in magmas and conservative behaviour in hydrothermal systems, remote measurements of carbon dioxide proportions and emission rates would be extremely valuable for monitoring, but they remain difficult because of its high atmospheric concentration.