This paper describes techniques which have been developed to analyse the gaseous and particulate emissions from diesel engines over the US Federal heavy duty transient test cycle. This has included identifying the main sources of emissions during the test cycle, and analysis of the type of particulate. Some of the interesting observations from recent test work have been reviewed together with solutions that have reduced emissions levels. A computer simulation using steady state results has been presented which correlates well with measured values over the transient test cycle. Finally the paper considers the measures required to approach 1991 and 1994 emissions levels.