In this article we present the structure and outline of the proposed content of the LKIF-Core ontology. LKIF is an interchange format for legal knowledge, under development in Estrella, a 6th framework European project. One of the layers of expressiveness in LKIF is to consist of a combination of the OWL-DL and SWRL knowledge representation languages. LKIF-Core adds an ontology containing definitions of basic legal concepts. A number of these concepts are used to define frameworks that capture their use in legal reasoning. LKIF-Core is intended to enable re-use in the construction of legal knowledge bases and as semantic grounding for the translation of existing knowledge formats into LKIF.