Any event that causes a significant displacement of the seafloor also causes the displacement of an equivalent volume of water. This is the basic mechanism governing the generation of tsunamis. Although most tsunamis are produced from earthquakes, they can also be caused by volcanic activity, submarine landslides, slumps, meteor impacts, and occasionally by human activity. The primary cause of wave generation is the release of energy and associated crustal deformation resulting from the earthquake. Thus, any earthquake that produces a tsunami is known as a tsunamigenic earthquake. The magnitude of the earthquake does not dictate whether or not a tsunami will be produced or its size; these issues are decided by the type of fault from which the earthquake is generated. The Mw 9.0 December 2004 earthquake is the largest to have occurred in the Andaman-Sumatra subduction zone. This chapter summarizes the findings of various scientific groups that investigated the immediate impacts of the December 26, 2004, tsunami along the coasts of the Andaman and Nicobar Islands, the Indian state in the Bay of Bengal, from field-based and remote-sensing studies. Wherever available, data prior to the December 2004 tsunami were compared to the changes after the event.