An integrated circuit is provided with a diagnostic data capture and output system in the form of a diagnostic data capture circuit which captures a data word and a context word from a bus. The bus may be the functional bus connecting functional circuits within the integrated circuit or a dedicated bus linking one or more functional circuits directly to the diagnostic data capture circuit. The diagnostic data captured is buffered within a first-in-first-out buffer and then serialised for output. The diagnostic data fields also include a time value indicative of the time at which the diagnostic data field concerned was captured and whether any diagnostic data fields have failed to be captured.