Marine diesel engines are in service with NOx emission levels far below the IMO limits. It is expected to use water emulsification for the uncontrolled NOx, and the engine should therefore be equipped with fuel pumps in order to allow for the larger volume fuel–water mixture to be injected. Therefore the other pollutants are also subject to limitations. Legislation restricting NOx emission levels to as low as 100 ppm or < 15% oxygen is being enforced in some local areas on land as well as in some coastal waters. It is believed that such levels may be more frequent in the coming years and may be enforced in local areas. NOx emissions of the above level from high efficiency engines can only be obtained by employing catalytic converters. The SCR process and reactor sizing confront the ship managers and cause them to rearrange the engine room space. Hence the reactor is an integral part of the engine exhaust and turbocharging system, and the influence of the reactor volume and heat capacity must therefore be carefully evaluated in terms of the control of the engine dynamics, especially during transient conditions such as load changes, startup, etc.