In this paper we describe a legal core ontology that is part of a generic architecture for legal knowledge systems, which will enable the interchange of knowledge between existing legal knowledge systems. This Legal Knowledge Interchange Format, is under development in the Estrella project and has two main roles: 1) the translation of legal knowledge bases written in different representation formats and formalisms and 2) a knowledge representation formalism that is part of a larger architecture for developing legal knowledge systems. A legal (core) ontology can play an important role in the translation of existing legal knowledge bases to other representation formats, in particular into LKIF as the basis for articulate knowledge serving. We describe the methodology underlying the LKIF core ontology, introduce the concepts it defines, and discuss its use in the formalisation of an EU directive.