Until recently, people living in a volcano's shadow had little help in predicting eruptions. In the past 300 years, volcanic eruptions, most of them unexpected, have killed more than 250,000 people. In 2000, experts estimated that 500 million people were living in areas at risk from catastrophic volcanic eruptions. This illustrated essay describes the strides scientists have made in eruption forecasting in recent years, especially tools for detecting changes in a volcano's seismicity, ground deformation, and gases, and explores why accurately predicting volcanic events remains difficult. A background essay and discussion questions are included.