A multiprocessor computer system which provides fault tolerance includes a number of processing sets. At least one of the processing sets is operable asynchronously of a second processing set. A monitor is connected to receive I/O operations output from the processing sets for identifying faulty operation of those units. The monitor is also operable to synchronise operation of the processing sets by signalling the processing sets on receipt of outputs from those units indicative of a plurality of them being at an equivalent stage of processing. The monitor provides for buffering of I/O operations output from the processing sets and for selective forwarding of those I/O operations to an external I/O bus. The processing set may be formed from a single processor or from multiple processors.