An intervention Autonomous Underwater Vehicle (AUV) is a significant step forward from a conventional AUV because tasks usually performed using a Remotely Operated Vehicle (ROV) are made possible. In addition, its versatility opens a range of new applications, e.g. black-box recovery. The successful execution of such a mission requires that the Navigation, Guidance and Control of an intervention AUV is capable of simultaneous localisation and mapping, target localisation, obstacle avoidance and robust supervisory operation. In this technical note we will seek to explore the ramifications of these requirements on the technologies, their integration, and mission control. A detailed discussion of all the facets of the mission requirements will be given, and a modular system presented. Furthermore, the concept of an AUV simulator will be introduced as a tool for the testing of navigation, guidance and control systems. Such a simulator environment would allow for complete mission scenarios to be tested.