Ocean acoustic tomography has become a highly developed tool for observing the ocean's interior. It has been used in the Atlantic, Pacific, and Indian Oceans, the Mediterranean, Greenland, and Norwegian Seas, and in a number of smaller bodies of water including Puget Sound. There are active ocean tomography research programs in the U.S., France, Germany, Japan, and India. In the same sense that x-ray, or ultrasonic acoustic tomography is used to image parts of the human body, ocean tomography provides images of the ocean's interior. This presentation will describe the methodology and present examples of tomographically derived ocean images.