The following article is motivated by the current efforts of the Object Management Group (OMG) to standardize object-oriented modelling languages. In the face of the great economic importance of such a standardization, a careful consideration of possible requirements for modelling languages is necessary. The development of suitable criteria and measures is, however, a delicate task. One reason for this is that the various tasks during the software engineering demand different, partly contradictory, requirements. Beyond this the valuation of quality is not independent from individual preferences and patterns of perception. In this article, we will give a framework for evaluating modelling languages. We will consider different levels of requirements - from subjective ones, like clarity of notation, to technical ones, like completeness and correctness of the language description. Finally we will use this framework for the comparison between OML and UML. An extended version of this compar...