Volcanic eruptions are impressive demonstrations of the activity of our planet. While some eruptions may be safely observed from distance, many eruptions, especially if explosive, may be hazardous to the populations and the environment around the volcano, including the animals, plants, and manmade structures. To reduce harm caused by eruptions, scientists called volcanologists attempt to forecast eruptions. Although volcanoes commonly provide several kinds of warnings before erupting, there is no single warning signal that allows volcanologists to accurately predict every eruption. Instead, data from various types of monitoring instruments are combined to help scientists forecast eruptions at least a few days in advance. In the next decades, as volcanologists improve monitoring systems and better understand the processes happening inside volcanoes, more precise eruption forecasts will be possible.