Presentation will discuss the options and equipment needed on-board to comply with Tier III of MARPOL Annex VI Regulation 13. Drastic reduction of allowable nitrogen oxides in the exhaust gases can only be achieved by EGR (exhaust gas recirculation) or SCR (selective catalytic reduction). Presentation will focus on the most promising SCR option, particularly on examination of the critical aspects of the installation, hints for equipment selection, degradation mechanisms of the catalyst as well as ways to improve performance. Key aspect of SCR is the need to have a high enough exhaust gas temperature for the reduction reaction to take place and to avoid formation of ammonium bisulfate ABS which is masking the catalyst. Measures must be taken during low load operation, associated with low exhaust gas temperatures. This problem appears on two-stroke engines which constitute the majority in ocean going vessels. Techno-economic data will also be included.