This article reviews the literature on drought over the last millennium, updates on global aridity changes from 1950 to 2008, and projected future aridity based on recent studies and model simulations. Historical droughts have occurred multiple times in North America, West Africa, and East Asia, often triggered by tropical sea surface temperature (SST) anomalies. Over Africa, recent Sahel droughts are linked to the southward shift of warm SSTs in the Atlantic and warming in the Indian Ocean. Local feedbacks can enhance and prolong droughts. Global aridity has increased significantly since the 1970s due to drying over Africa, southern Europe, East and South Asia, and eastern Australia. El Niño-Southern Oscillation (ENSO), tropical Atlantic SSTs, and Asian monsoons have played significant roles, but recent warming has also increased atmospheric moisture demand and altered circulation patterns. Climate models project increased aridity in the 21st century over most of Africa, southern Europe, the Middle East, the Americas, Australia, and Southeast Asia. Regions like the United States have avoided prolonged droughts in the last 50 years due to natural climate variations but may face persistent droughts in the next 20-50 years. Future efforts to predict drought will depend on models' ability to predict tropical SST variations.This article reviews the literature on drought over the last millennium, updates on global aridity changes from 1950 to 2008, and projected future aridity based on recent studies and model simulations. Historical droughts have occurred multiple times in North America, West Africa, and East Asia, often triggered by tropical sea surface temperature (SST) anomalies. Over Africa, recent Sahel droughts are linked to the southward shift of warm SSTs in the Atlantic and warming in the Indian Ocean. Local feedbacks can enhance and prolong droughts. Global aridity has increased significantly since the 1970s due to drying over Africa, southern Europe, East and South Asia, and eastern Australia. El Niño-Southern Oscillation (ENSO), tropical Atlantic SSTs, and Asian monsoons have played significant roles, but recent warming has also increased atmospheric moisture demand and altered circulation patterns. Climate models project increased aridity in the 21st century over most of Africa, southern Europe, the Middle East, the Americas, Australia, and Southeast Asia. Regions like the United States have avoided prolonged droughts in the last 50 years due to natural climate variations but may face persistent droughts in the next 20-50 years. Future efforts to predict drought will depend on models' ability to predict tropical SST variations.