The increase in the fuel price and more stringent regulations on greenhouse gases (CO2) make the engine compression ignition technology even more attractive in the context of internal combustion engines. This is because the modern turbocharged direct injection engines, with the common rail fuel system, are characterized by high combustion efficiency and power density, that make them particularly suitable both for applications on and off road.On the other hand, the compression ignition engines are subject to a heavy technological developments to meet the more stringent regulations on emissions of exhaust pollutants, especially PM and NOx. The adopted technologies have two main approaches, on the combustion and on the exhaust gas aftertreatment. The measures applied for combustion can reduce emissions, but with the risk of penalizing the other engine performances, such as noise, power output and fuel consumption. Instead, the technologies of exhaust gases treatment may allow the over coming of this conflict. In particular using a Selective Catalytic Reduction (SCR) catalyst, a NOx reduction can be reached at least up to 95%.The present article describes the activities to define and optimize a supply system for the SCR catalyst, applied to a wide range of diesel engines, from Light Duty (LD) vehicles to Off-Road (OR) applications. For this kind of applications the development has taken into account requirements in terms of performance, layout and cost, unlike the Heavy Duty (HD) vehicle, where the SCR is employed for many years with less constraints.The development phase has been characterized by a 0-D model simulation activity to define the system performances and the main management strategies, and 1-D and 3-D model simulation activities for the hydraulic design.