An instrumentation system for monitoring one or more valves of an assembly includes one or more sensors coupled to the one or more valves, and a data processing arrangement for processing sensor signals generated by the one or more sensors to provide an indication of operation of said one or more valves. The one or more sensors and the data processing arrangement are operable to compute one or more risk factors {R) from the sensor signals indicative of operating reliability and/or safety of the one or more valves. The instrumentation system is, for example, suitable for use with valves of the X-mas tree (Christmas tree) assembly for oil and gas production, for example in a sub-sea environment.