A data processing apparatus having one or more trace data sources. At least one of said trace data sources includes a trace data generator responsive to activity in monitored circuitry to generate trace data representing said activity. A synchronization marker generator is coupled to the trace data generator and operates to generate a synchronization marker and insert the synchronization marker into the trace data stream. A controller is coupled to the synchronization marker generator to generate and insert a synchronization marker into the trace data stream. The controller controls initiation in dependence on behavior of the data processing apparatus downstream of the trace data generator. In this way, the downstream behavior of the data processing apparatus can be made to influence the rate and timing of insertion of synchronization markers into a trace data stream.