The 2004 Indian Ocean tsunami was triggered by a 9. 15 magnitude earthquake (MELTZNER et al. , 2006; CHLIEH et al. , 2007) that occurred at 0:58:53 GMT, 7:58:53 LT (USGS) (t ). The epicenter was located at 3. 3 N, 95. 8 E (Fig. 1) with a focal depth of EQ approximately 30 km. The earthquake was responsible for a sudden fault slip estimated on average from 12-15 m (SYNOLAKIS et al. , 2005; LAY et al. , 2005) to 20 m (FU and SUN, 30 2006). The seismic moment estimate (Mo = 1. 3 5 9 10 dyne-cm), based on the Figure 1 Locations of video recordings, recovered clocks, and reliable eyewitness observations. 1: Coastal plains ?ooded by the tsunami; 2: non-?ooded coastal plains; 3: uplands. Insert 3D-map showing the Sumatra Island, the studied area, and the epicenter of the 26/12/2004 earthquake. The video taken at Uteuen Badeue, on the eastern edge of the Banda Aceh Bay, was recorded by the chief of the Fishery Regional Of?ce from the top of a cliff. The movie that was shot near the Baiturrahman mosque in downtown Banda Aceh has been shown worldwide on TV. The one at Peukan Bada has been recorded during a wedding party. The last two movies were analyzed in detail in order to calculate the tsunami velocity (FRITZ et al. , 2006). Vol.

The tragedy of the 2004 Indian Ocean tsunami has led to a rapid expansion in science directed at understanding tsunami and mitigating their hazard. A remarkable cross-section of this research was presented in the session: Tsunami Generation and Hazard, at the International Union of Geodesy and Geophysics XXIV General Assembly in Perugia, held in July of 2007. Over one hundred presentations were made at this session, spanning topics ranging from paleotsunami research, to nonlinear shallow-water theory, to tsunami hazard and risk assessment. A selection of this work, along with other contributions from leading tsunami scientists, is published in detail in the 28 papers of this special issue of Pure and Applied Geophysics: Tsunami Science Four Years After the Indian Ocean Tsunami. Part I of this issue includes 14 papers covering the state-of-the-art in tsunami modelling and hazard assessment. Another 14 papers are published in Part II focusing on observations and data analysis.