Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 140. Subaqueous explosive eruptions are common, and in earth′s early history were ubiquitous. Although they are unlike eruptions we find on land, they operate with the same fundamental processes. Deep–sea eruptions modify important seafloor hydrothermal systems and their coupled habitats for extremophile organisms, and large eruptions on the continental shelf presumably have as yet unknown effects on a wide range of marine organisms. Shallow eruptions that can affect shipping lanes and threaten coastal environments, either directly or by generation of tsunami, also appear to produce deposits and conditions closely linked with formation of significant chunks of the world′s mineral resources.