The human tragedy caused by the Sumatra-Andaman earthquake (moment magnitude 9.3) on 26 December 2004 and its companion Nias earthquake (moment magnitude 8.7) on 28 March 2005 is difficult to comprehend. These earthquakes, the largest in 40 years, have also left seismologists searching for the words and tools to describe the enormity of the geological processes involved. Four papers in this issue discuss aspects of a rupture process of surprising complexity, the first such event to test the sensitivity and range of many new technologies. A surprising feature of the earthquake is that after the initial rapid rupture, subsequent slip of the plate interface occurred with decreasing speed toward the north.