Det Norske Veritas (DNV) Maritime, Section for Machinery, is responsible for approval of machinery in ships for DNV Class. Allowable fatigue stresses in forged steel is a key input parameter for such evaluations, and are regulated in DNV’s Rules. It is well known that the cleanliness of the steel is important for its fatigue strength. To account for this, “clean steel” has been defined in DNV’s Rules for forgings. For “clean steel”, an extra 10% is allowed in the components fatigue strength, compared to ordinary forged steels. Clean steel is given a factor of 1.1. Recently, one leading steel manufacturer has claimed that even higher benefits can be gained by using “super clean steels”, and a factor 1.15 has been claimed. The improvement has been demonstrated by fatigue testing. It appears that although the claimed 125% may be justified, there are limits to how much gain it is possible to obtain by increasing steel cleanliness. In order to approve such increased fatigue strength in attempt has been made to find ways to correlate demonstrated cleanliness to increased fatigue strength by chemical and/or metallographic means without performing extensive fatigue test series. Only with such criteria, it is possible to certify “super clean steel”, and accept the use of its increased fatigue strength with reference to the material certificate. This paper covers a study concerning the methods that are available for determining the cleanliness level for steels. Examples of methods that can be used for finding and testing material for the presence of big inclusions are reported.