###### Abstract  
Development of Autonomous Underwater Vehicles (AUVs) has permitted the automatization of many tasks originally achieved with manned vehicles in underwater environments. Teams of AUVs designed to work within a common mission are opening the possibilities for new and more complex applications. In underwater environments, communication, localization, and navigation of AUVs are considered challenges due to the impossibility of relying on radio communications and global positioning systems. For a long time, acoustic systems have been the main approach for solving these challenges. However, they present their own shortcomings, which are more relevant for AUV teams. As a result, researchers have explored different alternatives. To summarize and analyze these alternatives, a review of the literature is presented in this paper. Finally, a summary of collaborative AUV teams and missions is also included, with the aim of analyzing their applicability, advantages, and limitations. Autonomous Underwater Vehicle(s); collaborative AUVs; underwater localization 20  
Before establishing a collaborative scheme for AUVs, the problem of localization and navigation must be addressed for every vehicle in the team. Traditional methods include Dead-Reckoning (DR) and Inertial Navigation Systems (INS) [7]. DR and INS are some of the earliest established methods to locate an AUV [8]. These systems rely on measurements of the water-speed and the vehicle's velocities and accelerations that, upon integration, leads to the AUV position. They are suitable for long-range missions and have the advantage to be passive methods--they do not need either to send or receive signals from external systems--resulting in a solution immune to interferences. Nevertheless, the main problem of them is that the position error growths over time--which is commonly known as accuracy drift--as a result of different factors such as the ocean currents and the accuracy of the sensors itself, which are not capable of sensing the displacements produced by external forces or the effects of earth's gravity. The use of geophysical maps to matching the sensors measurements is an alternative to deal with the accuracy drifts of the inertial systems. This method is known as Geophysical Navigation (GN) [9] and allows accomplishing longer missions maintaining a position error relatively low. However, there is a need for having the geophysical maps available before the mission, which is one the main disadvantages of GN along with the computational cost of comparing and matching the map with the sensors data. Acoustic ranging systems have been another common alternative for AUV navigation [10]. These systems can be implemented using acoustic transponders to locate an AUV in either global or relative coordinates. However, most of them require complex infrastructure and the cost of such deployments could be higher compared with other methods. In recent years, researchers are exploring new alternatives for AUV localization and navigation. Optical technologies have become very popular for robots and vehicles at land or air environments [11], but face tough conditions in underwater environments that have delayed the development of such technologies for AUVs. When the underwater conditions permit a proper light propagation and detection, visual-based systems can improve significantly the accuracy of the position estimations and reach higher data rates that acoustic systems. Finally, recent advancements in terms of sensor fusion schemes and algorithms are contributing to the development of hybrid navigation systems, which takes the advantages of different solutions to overcome their weaknesses. A sensor fusion module improves the AUV state estimation by processing and merging the available sensors data [12]. Some of the common sensors used for it are the inertial sensors of an INS, Doppler Velocity Loggers (DVL), and depth sensors.