The optimal control and safe operation of a nuclear power plant requires reliable information concerning the state of the process. Signal validation is the detection, isolation, and characterization of faulty signals. Properly validated process signals can provide increased plant availability and reliability of operator actions. A comprehensive signal validation software system has been developed for application to nuclear power plants. This system combines some previously established fault detection methodologies as well as some newly developed modules. The techniques have been implemented in a modular architecture that allows for the addition or removal of signal validation “modules” as deemed necessary. Intramodule confidence factors describing the validity of a given signal are derived using fuzzy membership functions. A final evaluation of signal status is made by the system executive based on results from each signal validation module. To make reliable decisions in this parallel system, a positive decision maker was developed. The hypercube signal validation methodology and the comprehensive system were tested using operational data from both a commercial pressurized water reactor and the Experimental Breeder Reactor II.