An apparatus for processing data is disclosed in which the operations of data processing circuitry are monitored by one or more trace data sources which generate items of trace data indicative of the data processing operations performed by the data processing circuitry. Trace data source identifiers in a resulting trace stream indicate the source of items of trace data and a selected trace data source identifier is included in the trace stream in response to a received flush request signal. All items of trace data generated before the apparatus received the flush request signal are included in the trace stream before the selected trace data source identifier, such that the conclusion of the response of the apparatus to the flush request signal can be identified.