Abstract: In a monitoring system for a message-based, multi-component software system, such as a distributed system, interface descriptions are stored electronically for the various interfaces. Messages transmitted or received at the interfaces can be monitored and their content decoded by reference to the stored interface descriptions. Raw message data can be logged. Either monitored or logged data can be viewed by the user, either in its raw form or decoded and presented for instance in a tabular or graphical form. Different monitoring behaviours of the system can be triggered by message content. Rules can be applied so that monitoring is focussed on selected message events and correlation can be done between message events at different interfaces. The monitoring system can be used with any software system for which interface descriptions are available.