Meta-data repositories are systems handling meta-data and meta-models natively. Such systems typically exhibit a layered meta-data achitecture. From a data-centric point of view the meta-meta-model of a repository system plays a fundamental role for structuring and organising the repository artefacts. In this paper we discuss the OMG Meta Object Facility (MOF), a meta-meta-model standard enjoying wide industry acceptance and serving as important component of the model driven architecture. In this paper we a comprehensive detour of MOF, we position it, explain its organisation through comprehensive examples and introduce the set of related technologies.