The Indian Ocean tsunami of 26 December 2004 shows poignantly that catastrophic tsunamis are too infrequent for their hazard to be characterized by historical records alone. Long-term geologic records provide opportunities to assess tsunami hazards more fully. Telltale deposits left by tsunamis help assess water depth and velocity of past inundations, estimate source locations, and aid in understanding how tsunamis affect the ecology and geomorphology of coastlines. Dated deposits allow estimates of times and recurrence intervals of past tsunamis. Such information guides mitigation efforts and may reduce losses from future tsunamis.