Advances in technology and increases in the geophysical instrumentation of volcanoes now allow the surface effects of subterranean volcanic processes to be measured in unprecedented detail. Parameters such as ground motion and atmospheric disturbance are continuously monitored at some volcanoes, with high-sensitivity instruments covering wide frequency ranges. The wealth and accuracy of the resulting data have permitted the identification of oscillatory behaviors within magmatic systems, and have further indicated that these behaviors are common. One of the challenges for volcanology is to fully utilize the diverse information recorded at the surface to understand more of the underground source processes, ultimately applying this understanding to improving eruption forecasts.