Many Diesel engine development programs concentrate almost exclusively on steady state investigations to benchmark an engines performance. In reality, the inter-action of an engine's sub-systems under transient evaluation is very different from that evident during steady state evaluation. The transient operation of a complete engine system is complex, and collecting test data is very demanding, requiring sophisticated facilities for both control and measurement.This paper highlights the essential characteristics of a Diesel engine when undertaking testbed transient manouevres. Results from simple transient sequences typical of on-road operation are presented.The tests demonstrate how transient behaviour of the engine deviates greatly from the steady state optimum settings used to control the engine. The operation of the EGR system and its interaction with other sub-systems, in particular VGT, has a significant effect on emissions, fuel consumption and driveability, highlighting the need for dynamic optimisation as an integrated system.