Present trends indicate that further development of the diesel engine is largely dependent on its association with an exhaust driven turbocharger. High specific power output from a turbocharged vehicle diesel engine may be limited by narrowing compressor flowrange and low turbocharger efficiency at high pressure ratio. Two-stage (series) turbocharging is a potential method of overcoming these limitations. The complexity of two-stage turbocharged engine with inter and aftercooling, the large number of components involved and the wide operational range of modern automotive diesels call for more work on this subject. Good engine torque back-up and transient response should be prime objectives. This work examines the performance of a two-stage turbocharged diesel engine matched at low speed to produce good torque back-up. The relative sizes of the two turbines were selected to ensure good overall system efficiency over the engine operating range. Furthermore, a waste gate was fitted between the turbines in order to help maintain a power balance between the high and low pressure turbochargers over the speed range of the engine. Two different arrangements of the turbocharging system were tested. first, the 'Conventional' arrangement where the waste gate outlet port is connected to the outlet exhaust pipe. Secondly, the 'Non-Conventional' arrangement where the waste gate outlet port A comprehensive model, based on the 'filling and emptying' concept, has been extended to predict the performance of the engine with the waste gate arrangements. Detailed analysis of the waste gate response as well as the effects of partial admission conditions on the performance of a twin-entry turbine are included in the model. The prediction results are used to study the feasibility of uprating a conventional, 11 bar maximum BMEP, turbocharged engine to about 24 bar maximum BMEP. An extensive experimental program, using a computer controlled test bed facility, was carried out to measure the steady state and transient engine performance. The results were used to validate the theoretical model and to compare engine performance with the two waste gate systems. Experimental and predicted data showed favourable agreement, in most cases within the accuracy of the measurements and the component characteristics of the engine system. Comparisons of different two-stage turbocharging systems showed that a law speed match in conjunction with a waste gate improved the engine performance particularly at low speed. Engine acceleration response was considerably better as a result of the good torque back-up achieved.