System Design Studies for two cases of engine performance monitoring are used to point out characteristics that have significant influence on the reliability of the information provided to maintenance personnel. Limitations in the information content of the measurements and system uncertainties can lead to engine performance monitoring systems that are not credible in operation. Prescription of achievable functions is essential to the definition of an effective engine performance monitoring system. The use of uncertainty analysis in the System Design phase assists in assessing these requirements. In particular, it can indicate the measurements needed to provide the required information. Uncertainty analysis should also be used in assessing the performance of the system during run time as it provides the user with an indication of the reliability of the information presented.