Abstract: A method for on-line monitoring and testing the behavior of middleware based, distributed application software during run-time of such software is disclosed. In order to automatize the monitoring of the behavior of the application software, the monitoring method comprises: defining events capturing the behavior of the software execution, the events being based on an abstraction of the application software, the abstraction being provided by middleware; using code generating means and an instrumentation technique for automatically adding code to the implementation of the software code suitable for generating traces suitable to be sent to an observer, the information carried by the added code including information on the order of occurrence of the events and on the application software part location where each event occurs; and using a monitoring mechanism based on sending of trace reports to the observer, which ensures or takes into account the time order of the reported traces.