Abstract: A method of tracking a system (1) having sensors (3) (e.g. gas turbine engines) by providing a computer model (4) which, from changes in sensor readings, can track (follow) changes in performance parameters of the system. These may be e.g. efficiencies of various of components such as compressors and turbines. In the common embodiment, the method used (Singular Value Decomposition) is a way of computing the most likely solution when there are more performance parameters than the number of sensors used and thus a degree of redundancy. The method allows system diagnosis and most importantly a tracked model allows the selection of a tailored control regime (9) which results in optimum efficiency for the individual system (engine).