Many techniques are used for determining positions for applications such as the navigation of remotely operated vehicles (ROV) and autonomous underwater vehicles (AUV) and the tracking of vehicles, divers and even dolphins. The best known techniques use arrays of hydrophones, beacon (also called pingers), transponders (receiver-transmitters) and responders (transmitter-receivers). There are also various sonar systems which may be used for tracking active or passive targets and other acoustic systems such as Doppler logs and correlation logs which measure the speed of a vessel or submersible and therefore are an aid to navigation by dead reckoning. The general principles of underwater navigation and tracking are described and details of three less well known topics are presented: dead reckoning, diver self-navigation and real time tracking of dolphins.