More than half of the world’s people inhabit the region of the Asian monsoon—a region where torrential rains, floods, storms, cold-air surges, droughts, and heat waves are a part of the economy, culture, and rhythms of life. Showing the strongest seasonal variations in the Earth’s climate system, the Asian monsoon has far-reaching impacts on global climate and the environment. Its variability exemplifies complex interactions among the Earth’s surface, ocean, atmosphere, and biosphere. Prediction of monsoon climate fluctuation and future change is one of the greatest scientific challenges and deepest societal concerns.