1. A diagnostics system comprising: a topological map of a target system that has nodes that correspond to components of the target system and links that correspond to connections between the components; a knowledge store that has a structure that reflects or corresponds to that of the topological map, the store having a plurality of sections or libraries each of which is provided for storing data associated with one of the nodes defined in the topological map; means for receiving data from one or more sensors on the target system; means for including either the received data in the topological map and/or data that is a function of that received data, and means for diagnosing faults using the data in the topological map and the knowledge store.