Testing virtualised network functions

A system can comprise several virtual network functions (VNF) 150, 160, 170 which provide heterogeneous functions. The can be several 'component' instances (e.g. 152, 154, 156) of each VNF. The invention tests whether combinations of instances of different functions perform as expected during commissioning. In one claim, an instance 152 in one function is instructed to directly interoperate with and instance 162 in a different function. In another claim, instances in functions providing an end-to-end service are configured with test configurations. A further claim to checking the availability of resources before instantiating an NFV instance is also present.