Background InformationRetrieval (IR) has become, mainly as aresultofthe huge impact of the World Wide Web (WWW) and CD-ROM industry, one of the most important theoretical and practical research topics in Information and Computer Science. Since the inception ofits first theoretical roots about 40 years ago, IR has made avariety ofpractical, experimental and technological advances. It is usually defined as being concerned with the organisation, storage, retrieval and evaluation of information (stored in computer databases) that is likely to be relevant to users' informationneeds (expressed in queries). A huge number ofarticles published in specialisedjournals and at conferences (such as, for example, the Journal of the American Society for Information Science, Information Processing and Management, The Computer Journal, Information Retrieval, Journal of Documentation, ACM TOIS, ACM SIGIR Conferences, etc. ) deal with many different aspects of IR. A number of books have also been written about IR, for example: van Rijsbergen, 1979; Salton and McGill, 1983; Korfhage, 1997; Kowalski, 1997;Baeza-Yates and Ribeiro-Neto, 1999; etc. . IR is typically divided and presented in a structure (models, data structures, algorithms, indexing, evaluation, human-eomputer interaction, digital libraries, WWW-related aspects, and so on) thatreflects its interdisciplinarynature. All theoretical and practical research in IR is ultimately based on a few basic models (or types) which have been elaborated over time. Every model has a formal (mathematical, algorithmic, logical) description of some sort, and these decriptions are scattered all over the literature.