The paper deals with software sensor design for unmeasurable variables of anaerobic digestion processes. For this purpose, different mathematical models of anaerobic digestion and different theoretical approaches (differential algebraic approach, Kalman filter modifications and H-infinity filter) have been applied to develop software sensors as dynamic relations between some easily measurable variables and some unmeasurable ones (specific growth rates, biomass and substrate concentrations) of the most important bacteria participating in these processes. Comparative studies have been provided via computer simulations. The experimental validation of the sensors is under investigation on a pilot-scale anaerobic bioreactor with computer monitoring system.