Regulation of modern societies requires the generation of large sets of heterogeneous legal documents: bills, acts, decrees, administrative decisions, court decisions, legal opinions, circulars, etc. More and more legal publishing bodies publish these documents online, although usually in formats that are not machine-readable and without following Open Data principles. Until an open by default generation and publication process is employed, ex-post transformation of legal documents into Legal Open Data is required. Since manual transformation is a time-consuming and costly process, automated methods need to be applied. While some research efforts toward the automation of the transformation process exist, the alignment of such approaches with proposed Open Data methodologies in order to promote data exploitation is still an open issue. In this paper, we present a methodology aligned to the Open Data ecosystem approach for the automated transformation of Greek court decisions and legal opinions into Legal Open Data that builds on legal language processing methods and tools. We show that this approach produces Legal Open Data of satisfying quality while highly reducing the need for manual intervention.Information 2020, 11, 10 2 of 30 manual analysis and processing is prohibitive and automated approaches are needed to undertake such tasks [5]. It is noteworthy that some researchers explicitly classify legal documents as Big Data since they meet at least two of the major aspects of Big Data: volume and variety [6]. While several governments are taking steps to apply semantic web technologies in the legal publishing domain (e.g., the legislation.gov.uk platform where UK legislation is published in XML and RDF) [7], in most countries these documents are usually made available in unstructured or poorly structured formats; for example, as PDF files, HTML documents, or plain text.On the other hand, the current trend of Open Data requires that government data (legal documents being a special category of them) are published without technical and legal impediments, in structured and machine-processable formats, and under an open license. Janssen et al. [8] have discussed in detail the expected benefits from the adoption of the Open Data model: transparency, democratic accountability, economic growth, stimulation of innovation, creation of new services, improved decision and policy making, equal access to data, etc. To ensure these benefits of Open Data, researchers propose the "transparency-by-design" model, an organization model for data publication systems, which is expected to contribute to the automatic opening of data [9]. However, this is not yet the case in the legal domain, and the ideal scenario, where legal publishing systems are organized in such a way that legal information is generated in the form of Open Data by default, still seems distant. The availability of data in non-machine-processable formats, and the need for manual processing and conversion are identified as significant barriers for the use of Open Dat...