The International Maritime Organization (IMO) has been working on the reduction of emissions in sea shipping. This has been done by stricter regulations over the past years in the form of emission limits. An important element which determines the success of this reduction, is the enforcement of the emission limits. A drawback of the current enforcement procedure is that there is insufficient insight in what happens at open sea or, for example, at the borders of an Emission Control Area (ECA). TNO investigates if there are methods to improve the enforcement of emissions in shipping. Therefore, TNO started a research into an on board emission monitoring system. This report is the first step in this research and has the objective to recommend an on board monitoring system that is able to monitor pollutant emissions of seagoing vessels. The polluting emissions of seagoing vessels that are investigated in this report are: sulphur oxides (SOx ), nitrogen oxides (NOx ), particulate matter (PM) and black carbon (BC). The legislation study showed that there is no legislation for an on board emission monitoring system. This means that there is no obligation for the system in the near future. However, the IMO and the European Union want to investigate enforcement options in the future, which offers possibilities for the emission monitoring system. The available on board monitoring systems were divided into two groups, the low-end systems and the high-end systems. It turned out that the low-end systems are not yet sufficiently developed to withstand the maritime conditions and could therefore not be used as an emission monitoring system in the near future. The high-end systems showed to be suitable in the near future as an on board monitoring system. Therefore, the high-end systems were compared and assessed with each other on characteristics like: robustness, costs, accuracy, number of sample points, dimensions, emission measurement and maintenance. The assessment of the high-end systems was executed in the form of a multiple-criteria decision analysis. The on board emission monitoring system that scored the best was the Opsis M800 from Consilum. The main advantage of this system is that it operates with the UV/IR Differential Optical Absorption Spectroscopy principle. The big advantage of this principle is that the sensors and the exhaust gas are separated, which causes much less maintenance. The above mentioned on board emission monitoring system is therefore recommended, which means that the objective of this research is met. To improve this recommendation it will be desirable and recommended to test all the high-end systems in practice in order to determine and compare their performance for the above mentioned criteria. The opinion from the stakeholders is that they see potential in the system, especially with the task to create a level playing field, after the implementation of the 2020 sulphur limit. However, they also indicate that international legislation will be necessary to implement the system successfully. The shipping companies also indicated that they see potential for the on board emission system besides the enforcement task.