Apparatus comprising logic analyzer circuitry comprises a succession of two or more successive trigger condition detectors each configured to detect a match between a respective trigger condition and data handling activity relating to data handling transactions each having a respective transaction identifier; the succession of trigger condition detectors being configured so that a detection by a trigger condition detector of a match with its respective trigger condition enables a next trigger condition detector in the succession to initiate detection of a match with the respective trigger condition of that next trigger condition detector; and a transaction identifier detector associated with a first trigger condition detector in the succession, configured to detect the transaction identifier relating to a data handling transaction for which a match is detected by the first trigger condition detector, and to supply the detected transaction identifier to a subsequent trigger condition detector in the succession of trigger condition detectors; in which the subsequent trigger condition detector is configured to apply the detected transaction identifier as at least a part of its respective trigger condition so as to detect a match only in respect of a data handling transaction having that transaction identifier.