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High power density, high reliability and good controllability for varying load requirements are typical characteristics of hydraulic drive systems used for power transmission in stationary and mobile applications. Furthermore, hydraulic systems are usually safety related systems. Consequently, the handling of uncertainties in hydraulic systems is essential. A major source of uncertainty is, in particular, the wear induced change of the system behavior. Nowadays, the most common way to face uncertainty is the oversized system design to ensure reliable system operation. However, the uncertainty remains. In the first part of the paper we present a general approach to face uncertainty by means of a soft sensor network. Soft sensor networks make it possible to gather system information redundantly. In this way the occurrence of data conflicts are allowed which serve as an indicator of uncertainty. The resolution of these conflicts either lead to increased confidence for the model-based system information or allows the detection of changing component characteristics. The application of a soft sensor network to a hydraulic drive system is illustrated and discussed in the second part of this paper.