Root cause analysis for complex event processing is described. In embodiments, root cause analysis at a complex event processor is automatically performed by selecting an output event from an operator and correlating the output event to an input event using event type and lifetime data for the input event and the output event stored in a data store. Embodiments describe how the lifetime data can comprise a start time and an end time for the event, and the correlation can be based on a comparison of the start and end times between the input and output events. Embodiments describe how the correlation algorithm used is selected in dependence on the event type. In embodiments, a complex event processing engine comprises a logging unit arranged to store in the data store an indicator of an event type and lifetime data for each output event from an operator.