1. A multi-stage event detector for monitoring a system to detect the occurrence of multi-stage events in the monitored system, the multi-stage event detector including: one or more event detecting detector units for detecting observable events occurring on the monitored system; one or more parameter generating detector units for generating parameter values which vary over time dependent on the behaviour of the monitored system; a hidden state determiner for determining a likely sequence of states of interest of the system based on the outputs of the one or more event detecting detector units; and a transition determiner for determining a likely transition occurrence based on a comparison of a set of values of a parameter or set of parameters generated by one or more of the one or more parameter generating detector units with a plurality of pre-specified functions or sets of values of a corresponding parameter or set of parameters associated with different transition occurrences.