The human casualties and socio-economic damage associated with the Great Sumatra-Andaman Earthquake of 26 December 2004 and the resulting Indian Ocean tsunami are discussed. The Sumatra-Andaman earthquake was the largest earthquake to occur since the advent of global digital seismometry and it produced the most devastating tsunami in recorded history (and the largest humanitarian response). A reliable Indian Ocean Tsunami Warning and Mitigation System is shown to require an improved seismographic network, a real-time sea-level observing network covering the entire Indian Ocean basin, and the deployment of deep-ocean pressure sensors. It is concluded that Indian Ocean governments can achieve more tsunami-resilient communities by addressing poverty, promoting education, harnessing technological advances, investing in emergency medical and rescue services, and empowering stakeholders.