Abstract: A virtual sensor based monitoring and fault detection/classification system (10) for semiconductor processing equipment (12) is provided. A plurality of equipment sensors (14) are each operable to measure a process condition and provide a signal representing the measured process condition. A plurality of filtering process units (16) are each operable to receive at least one signal from the plurality of equipment sensors (14) and to reduce data represented by the at least one signal and provide filtered data. A plurality of virtual sensors (24) are each operable to receive the filtered data. The plurality of virtual sensors (24) model states of the processing equipment (12) and a work piece in the processing equipment (12). Each virtual sensor is operable to provide an output signal representing an estimated value for the modeled state.