The Landoldt-Börnstein Data Collection has become known as a compilation of numerical data and functional relations. However, already in the past some volumes have been published which went beyond that objective and provided a more comprehensive summary of a special field. In agreement with the Editor-in-Chief, Prof. Werner Martienssen, Volume I/21 will be a further step in extending the purpose and in modernizing the Landoldt-Börnstein Series. This volume will provide in the style of an encyclopedia a summary of the results of particle physics and the methods and instruments to obtain this information. Subvolume I/21A reports on the present state of theoretical and experimental knowledge in particle physics. In Subvolume I/21B detectors and data handling will be covered and Subvolume I/21C will be devoted to the technology of accelerators and colliders. The time to give a summary of elementary particle physics seems to be appropriate. The results of the electron-positron collider LEPat CERN have been fully analyzed and also the data of the other facilities in operation (e.g. TEVATRON and "Beauty-factories") have achieved a certain maturity. It will take several years before new data from the LHC starting operation in 2008 will become available to indicate new ways beyond the well established "Standard Model of Particle Physics". Of course, results from n- accelerator physics continue to come in and the most recent results are reported. Since particle physics and cosmology become ever more intertwined one chapter is devoted to this topic.