Volcanic degassing represents an essential component of the global geochemical cycles that determine the state of the atmosphere and climate. It also exerts a first-order influence on the ways in which volcanoes erupt and is thus vital to understanding how volcanoes work and assessing their hazards. This article reviews the sources of volcanic volatiles, their behavior in ascending magmas and surficial reservoirs (including isotopic fractionation), and the processes by which gases separate from melt and reach the atmosphere. A range of field, laboratory, and remote sensing techniques can be applied to measurements and monitoring of volcanic gas and aerosol compositions (elemental, molecular, and isotopic) and flux. It summarizes their application and the general characteristics of volatile emissions associated with different geodynamic settings and volcanic manifestations. Some methods, including those based on petrological and ice core analysis, can even provide estimates of volatile budgets of eruptions that occurred in the distant past. Having considered the source-to-surface processes related to volcanic degassing, it reviews the impacts of emissions on the atmosphere, climate, and environment, from local to global scales, and the associated human and animal health hazards of volcanogenic pollution.