The need to meet the US 2007 emissions legislation has necessitated a change in Diesel engine technology, particularly to the fuel injection equipment (FIE). At the same time as these engine technology changes, legislation has dictated a reduction in fuel sulphur levels and there has also been increased use of fatty acid methyl esters (FAME) or biodiesel as a fuel blending component. The combination of changes to the engine and the fuel has apparently led to a sharp rise in the number of reports of field problems resulting from deposits within the FIE. The problem is usually manifested as a significant loss of power or the engine failing to start. These symptoms are often due to deposits to be found within the fuel injectors or to severe fouling of the fuel filter. The characteristics of the deposits found within different parts of the fuel system can be noticeably different. A variety of analytical techniques have been investigated to gain knowledge of the characteristics of these different forms of deposit. Work has also been performed to characterise some of the fuels that may be causing the deposits.This paper concentrates on the characterisation of deposits found specifically in the fuel injectors. Deposits found within different parts of the injector have been analysed using, Gas Chromatography with Mass Spectrometry detection (GC/MS), Fourier Transform Infra-red analysis (FTIR), Inductively Coupled Plasma spectroscopy (ICP), Nuclear Magnetic Resonance spectroscopy (NMR) and elemental analysis. Fuel samples that have been associated with the deposit formation have also been analysed. The techniques discussed are high resolution mass spectrometry, and ICP. The results are also placed in context with previously published work on both filter and injector deposits.