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A key problem with autonomous underwater vehicles is being able to navigate in a generally unknown environment. The available underwater sensor suites have a limited capability to cope with such a navigation problem. In practice, no single sensor in the underwater environment can provide the level of accuracy, reliability and the coverage of information necessary to perform underwater navigation. Therefore there is a need to use a number of sensors and combine their information to provide the necessary navigation capability in a synergetic manner. This may be achieved by employing multisensor data fusion (MSDF) techniques and these are the subject of the material presented in this paper. INTRODUCTIONT he oceans cover 70% of the Earth's surface and contain an abundance of living and non-living resources that remain largely untapped waiting to be discovered. How ever, a number of complex issues, mainly caused by the nature of underwater environments, make exploration and protection of these resources difficult to perform. In the past few decades, various world-wide research and development activities in underwater robotic systems have increased in order to meet this challenge. One class of these systems is tethered and remotely operated and referred to as remotely operated vehicles (ROVs). Extensive use of ROVs is currently limited to a few applications because of very high operational costs and the need for human presence in conducting a mission. The demand for a more sophisticated underwater robotic technology that minimises the cost and eliminates the need for human operator and is therefore capable of operating autonomously, becomes apparent. These requirements led to the development of autonomous underwater vehicles (AUVs).To achieve truly autonomous behaviour, an AUV must be able to navigate accurately within an area of operation. In order to achieve this, an AUV needs to employ a navigation sensor with a high level of accuracy and reliability. However, in practice, as will be discussed in the next section, a single sensor alone may not be sufficient to provide an accurate and reliable navigation system, as it can only operate efficiently under certain conditions or it has inherent limitations when operating in underwater environments. It is therefore necessary to use a number of sensors and combine their information to provide the necessary navigation capability. To achieve this, a multisensor data fusion (MSDF) approach, which combines data from multiple sensors and related information from associated databases, can be used.The aim of this paper is to survey previous work and recent development in AUV navigation and to introduce MSDF techniques as a means of improving the AUV's navigation capability. The structure of this paper is as follows: the next section describes the navigation systems that are currently being used in AUVs. MSDF is then discussed, whilst MSDF using specific sensor combinations applied to the navigation of AUVs follow. AUTONOMOUS UNDERWATER VEHICLE NAVIGATIONNavigation systems used b...

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