Dormant since 1857, Mount St. Helens Volcano in southwestern Washington stirred from its repose to erupt on March 27, 1980, following a week of premonitory earthquake activity. The eruption was the first in the conterminous United States since the 1914-1921 activity of Lassen Peak, California. The eruptive activity through May 17 was intermittent and relatively mild, but the accompanying seismic activity remained intense. On May 18, a catastrophic eruption, triggered by a magnitude 5.0 earthquake, produced a massive landslide/debris avalanche, a devastating lateral "blast," pyroclastic flows, mudflows, and an ash column that rose more than 20 km into the stratosphere. Winds carried the ash easterly, and more than 7 cm of ash was deposited locally in parts of eastern Washington. The landslide/debris avalanche and associated mudflows caused flooding of the Toutle and Cowlitz River valleys, which carried sediment as far as the confluence with the Columbia, where it choked off the channel to navigation. Smaller but significant explosive eruptions followed in May, June, July, August, and October, 1980, with lava domes being extruded in the crater following the June, August, and October eruptions. Subsequently in December 1980 and February 1981, lava domes were extruded without significant preceding explosive activity. Except for the latter two, each dome was partly or wholly destroyed by succeeding explosive events. Scientists expect similar activity to continue for months or years--possibly even decades. The Mount St. Helens eruptions severely tested the ability of scientists to respond swiftly and effectively in assisting public officials during a geologic disaster. At the same time, they shall continue to provide an unprecedented opportunity for the systematic investigation of volcanic phenomena, and hopefully, the insight to meet possible future eruptions there and elsewhere in the Cascade Range with equal success.