This thesis describes the theoretical work completed in an investigation into the computer modelling of heavy vehicle power train systems and the associated experimental validation techniques used to verify the developed models. The theoretical work includes the development of computer models of individual driveline components and various techniques used to link the separate models to form a complete power train program. Both the steady state and transient performance of all components are considered: in particular the simulation of power train gearshifts are made which include the prediciton of diesel engine and fluid coupling performance during the brake band slipping mode. The experimental work has included the establishment of a complete vehicle driveline test facility for both steady state and transient performance measurements; in particular, driveline performance during gearshifts and the application of step loads.