This paper shows how techniques devised for aspect-oriented modelling (AOM) can be used for aspect-oriented metamodelling. Metamodelling is an approach to language definition, which has been adopted by industry for defining modelling languages, in particular the Unified Modelling Language (UML), and involves the construction of an object-oriented model of the abstract syntax and, optionally, the concrete notation and semantics of the target language. As the importance of models in software development grows, so does the importance of metamodels, not least to provide the basis of implementations of modelling tools. Specifically, there is a need for complete and accurate metamodels, which have a uniform architecture and which can be organized appropriately to support the definition of families of languages. The approach described in this paper uses AOM techniques to address this need. It is illustrated by extracts from the 2U submission to the UML 2.0 RFPs issued by the Object Management Group in 2001. The paper concludes with a discussion on the customization and generation of tools from such definitions.