A testing system for testing the radio software stack (40) of a radio device (16) is described. The system comprises test control means (10) and perturbation means (38) connected to said radio device. The perturbation means enables perturbation and subsequent monitoring of a radio stack operation. The perturbation means may also directly alter data in a radio message en-route through the software stack. In a further aspect of the present invention a distributed testing system is described enabling a wider range of environmental and real world situations to be simulated to enable thorough testing of the software stack in design for an application.