The 26 December 2004 magnitude 9 earthquake off Sumatra provided the first examples of travelling tsunami waves in mid-ocean clearly detected by satellite altimetry. The earthquake was the largest since satellite altimetry started in the 1970s and gave peak-to-trough wave heights in mid-ocean of over a metre. The tsunami was detected by three of the four altimeters presently giving sea surface height information. Each detected the spreading front twice, as it moved south-westwards into the Indian Ocean and as it moved northwards into the Bay of Bengal. They also detected the disturbed region closer to the epicentre that expands with the slower velocities of higher-frequency waves. Although the plate rupture is estimated to extend over about 1300 km in a north/south direction, the satellite observations appear consistent with a smaller generation area towards the south of this rupture zone. Fronts observed in the Indian Ocean show a positive first crest. Those observed in the Bay of Bengal are of smaller amplitude and appear to show a first negative first crest (trough). The structure in the Indian Ocean front observed by Jason-1 suggests the possible presence of a shorter-wavelength negative component superimposed on the positive crest.