Volcanic eruptions are one of Earth^s most dramatic and violent agents of change. Not only can powerful explosive eruptions drastically alter land and water for tens of kilometers around a volcano, but tiny liquid droplets of sulfuric acid erupted into the stratosphere can change our planet^s climate temporarily. Eruptions often force people living near volcanoes to abandon their land and homes, sometimes forever. Those living farther away are likely to avoid complete destruction, but their cities and towns, crops, industrial plants, transportation systems, and electrical grids can still be damaged by tephra, lahars, and flooding. Mount St. Helens was known as "the Fuji of America" because its symmetrical beauty was similar to that of the famous Japanese volcano. The graceful cone top, whose glistening cap of perennial snow and ice dazzled the viewer, is now largely gone. On May 18, 1980, the missing mountaintop was transformed in a few hours into the extensive volcanic ash that blanketed much of the Northwestern United States and into various other deposits closer to the mountain. Volcanic activity since 1700 A.D. has killed more than 260,000 people, destroyed entire cities and forests, and severely disrupted local economies for months to years. Even with our improved ability to identify hazardous areas and warn of impending eruptions, increasing numbers of people face certain danger. Scientists have estimated that by the year 2000, the population at risk from volcanoes is likely to increase to at least 500 million, which is comparable to the entire world^s population at the beginning of the seventeenth century! Clearly, scientists face a formidable challenge in providing reliable and timely warnings of eruptions to so many people at risk. When is a volcano considered active? Surprisingly, there is no consensus among volcanologists on how to define an active volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth^s volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of activity. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless but do not actually erupt. Are these volcanoes active? Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It^s important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawai`i, little more than 200 years. Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again. Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. For example, since calderas have lifespans sometimes measured in millions of years, a caldera that hasn^t produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Yellowstone caldera in Yellowstone National Park is at least 2 million years old and hasn^t erupted for 70,000 years, yet scientists do not consider Yellowstone as extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system, and rapid rates of ground uplift, many scientists consider it to be a very active volcano! A volcano may begin to show signs of unrest months to years before it erupts, but a warning that specifies when it will erupt months to years ahead of time is extremely rare. Just as important as an advance warning for saving lives and minimizing property damage, however, is the immediate detection of a sudden eruption or lahar and real-time notification of the activity to the public and local, state, and federal emergency-managment officials. USGS photo by Lyn Topinka, May 1982.