Underwater sensor networks (UWSNs) are mobile ad hoc networks that present the opportunity for many applications, such as coast surveillance, four-dimensional monitoring such as for ocean and biology studies, and oil and gas field monitoring. The chapter presents the current state of the art of UWSNs. Mobile underwater ad hoc networks are composed of sensor nodes that possess communication capabilities. Localization with autonomous underwater vehicles (AUVs) uses three messages: wakeup, request, and reply. The AUV sends the wakeup message to notify its presence to the nodes that are inside its communication range. In long-term applications, the usual localization approach has been the Long Base-Line technique. In these scenarios, equipments are placed kilometers apart and they collect data individually. Localization with AUVs uses three messages: wakeup, request, and reply. The AUV sends the wakeup message to notify its presence to the nodes that are inside its communication range.