This book includes papers from the twentieth JURIX conference (first organized in 1988). Over the years JURIX has become more and more international. JURIX is originally a Dutch/Belgian initiative. Nowadays, the conference papers are in majority from non-Dutch authors, and since 2002 JURIX is held outside the Netherlands and Belgium every other year. Most accepted papers can largely be fitted into either work on argumentation or work on ontology. Argumentation has been a JURIX-topic during all past years, and the interest in ontology has revived recently with Semantic Web initiatives. Topics include Case law, which is approached from different angles, and Automatic Detection of Argumentation in Legal Cases. A contribution is included on recommending techniques, which are novel for the field. Case-based reasoning has been explored in previous volumes and this year, CBR is integrated with argument schemes in two papers. In the short papers various topics are covered: ontology of the Polish Commercial Companies Code, a methodology for modeling legal workflows, content management and version management of legislation, representation of deadlines, and an ontology for summarizing documents.