The introduction of stringent EPA 2015 regulations for locomotive / marine engines and IMO 2016 Tier III marine engines initiates the need to develop large diesel engine aftertreatment systems to drastically reduce emissions such as SOx, PM, NOx, unburned HC and CO. In essence, the aftertreatment systems must satisfy a comprehensive set of performance criteria with respect to back pressure, emission reduction efficiency, mixing, urea deposits, packaging, durability, cost and others.For on-road and off-road vehicles, urea-based SCR has been the mainstream technology to reduce NOx emissions. For category II marine engines with single cylinder displacement volumes between 7 liters and 30 liters, IMO III (Tier IV) emission regulations dictate approximately 80% reduction of NOx emissions vs. Tier II emission regulations . Urea / ammonia SCR is being considered as an enabling technology to achieve IMO III regulations without significant impacts on engine performance and fuel economy.As always, engine OEMs attempt to develop technologies mostly via engine measures such as EGR and high pressure common rail, in effect minimizing the content of aftertreatment systems. For instance, one engine OEM announced that engine-only measures without aftertreatment devices can meet IMO III regulations . However, the efforts taken in engine developments are often enormous, requiring long development duration to resolve major technical challenges.Compared with land-based (including mobile) applications, large engine applications face multiple challenges. Due to high sulfur content in diesel fuel, sulfur reduction would be an important task, affecting fuel switching, sulfur scrubber choices, and aftertreatment layout designs. For instance, the wet / dry desulfurization with appropriate reheating mechanisms will affect SCR temperature window which typically operates between 250 °C and 450 °C .The scope of this paper is confined to urea SCR technology applications, hence does not touch complexities in sulfur related design issues. A typical urea SCR system includes a urea / ammonia injector / nozzles, injector housing, mixer, and appropriate pipe configurations. In marine applications, urea mixing, urea deposits, catalyst poisoning, dust removal, and catalyst choices are quite different from those of land-based mobile applications, however in many ways similar to power plant emission controls.Compared with land-based mobile applications, urea mixing is made more difficult because of large spatial size and mixing space; in addition, low flow rate and temperature tend to negatively affect urea evaporation and turbulence intensity. Urea deposits, as a result of incomplete evaporation of urea solution, can create concerns of backpressure, engine power loss and material deteriorations. Catalyst poisoning occurs due to solid chemical deposits (such as ammonium bisulfate) on catalyst surface as a result of the reactions between ammonia and sulfur. Both urea deposits and sulfates can be removed by higher temperature. Ash and dusts are much more severe in marine applications than in land-based mobile applications because of impurities (mostly sulfur) from marine diesel fuel (DFO, IFO, HFO), therefore care must be taken in selecting SCR catalysts.A successful urea SCR system design needs to satisfy a comprehensive set of performance criteria as outlined below: 1Efficient catalyst usage (small size)2High NOx conversion efficiency3No ammonia slip4No urea deposits5Low backpressure6Compact, low cost, and low weight7High acoustic (noise reduction) performanceThis paper briefly reviews existing SCR technology in marine applications. Then a specific case study is introduced with an initial design layout of urea SCR. CFD method was applied to simulate urea spray transport, evaporation, and droplet-wall phenomena. Engine dynamometer tests were performed to validate the initial design. The urea deposit locations from tests were compared with CFD predicted results. With gained insights, three geometrical configurations of urea SCR systems were studied to address the deposit concerns. Multiple influencing factors such as wall temperature and mixers are evaluated. The optimized design is summarized at the end. Future developmental directions on marine applications are recommended in the conclusion section.