In a method for testing a software system that operates a technical system including a technical system that is in operation by a customer, an operation model is generated that is based on a reproducible operation procedure under conditions relevant for the customer, and the operation of the technical system is monitored. The software system to be tested on the technical system of the customer is installed, and this technical system is operated and monitored with the operation model. Subsequently the operation of the technical system of the customer is evaluated with the operation model with regard to possible faulty executions.