Underwater Robots reports on the latest progress in underwater robotics. In spite of its importance, the ocean is generally overlooked, since we focus more of our attention on land and atmospheric issues. We have not yet been able to explore the full depths of the ocean and its resources. The deep oceans range between 19000 to 36000 feet. At a mere 33-foot depth, the pressure is twice the normal atmospheric pressure of 29.4 psi. This obstacle, compounded with other complex issues due to the unstructured and hazardous environment, makes it difficult to travel in the ocean even though today's technologies allow humans to land on the moon. Only recently, we discovered by using manned submersibles that a large amount of carbon dioxide comes from the sea-floor and that extraordinary groups of organisms live in hydrothermal vent areas. On March 24, 1995 Kaiko (a remotely operated vehicle) navigated the deepest region of the ocean, the Mariana Trough. This vehicle successfully dived to a depth of 33000 feet and instantly showed scenes from the trench through a video camera. New tools like this enable us to gain knowledge of mysterious places. However, extensive use of manned submersibles and remotely operated vehicles is limited to a few applications because of very high operational costs, operator fatigue and safety issues. In spite of these hindrances, the demand for advanced underwater robot technologies is growing and will eventually arrive at fully autonomous, specialized, reliable underwater robotic vehicles. Underwater Robots is an edited volume of peer-reviewed original research comprising thirteen invited contributions by leading researchers. This research work has also been published as a special issue of Autonomous Robots (Volume 3, Numbers 2 and 3).