Diesel fuel injector deposits have been observed in the field for many years. Their location and composition is dependant on the type of Fuel Injection Equipment (FIE) technology utilised in vehicles and fuel quality. This paper first characterises such deposits and then defines the maximum acceptable level to ensure best field performance for the following FIE systems: InDirect Injection (IDI), High Pressure Common Rail (HPCR) and Electronic Unit Injection (EUI).HPCR has been instrumental in achieving the lowest possible emissions and Fuel Consumption (FC) levels. It is now the only choice for new Passenger Car (PC) applications meeting Euro 5, US Tier 2 and Japan Post New Long Term emission regulations. Its use is also increasing in Heavy Duty (HD) applications. However, HPCR and EUI have both been shown to have a tendency to form injector deposits that can negatively impact emissions, power and fuel consumption. This has been confirmed in a variety of tests ranging from industry recognised bench engine tests to bench engine tests run in co-operation with OEMs and field programmes.The data developed show that appropriate Deposit Control Additive (DCA) technology can prevent and restore the FIE to its optimum operating conditions. This has been confirmed using a mix of reference and market relevant fuels.Any fuel additive solution should be harm free in field applications. Therefore, the DCA technology utilised in this paper has been validated through a series of robust harms tests that include a mix of standard industry tests, in house tests and field trials at elevated treat rates.