This new volume, I/23, of the Landolt-Börnstein Data Collection series continues a tradition inaugurated by the late Editor-in-Chief, Professor Werner Martienssen, to provide in the style of an encyclopedia a summary of the results and ideas of Relativistic Heavy Ion Physics. Formerly, the Landolt-Börnstein series was mostly known as a compilation of numerical data and functional relations, but it was felt that the more comprehensive summary undertaken here should meet an urgent purpose. Volume I/23 reports on the present state of theoretical and experimental knowledge in the field of Relativistic Heavy Ion Physics. What is meant by this rather technical terminology is the study of strongly interacting matter, and its phases (in short QCD matter) by means of nucleus-nucleus collisions at relativistic energy. The past decade has seen a dramatic progress, and widening of scope in this field, which addresses one of the chief remaining open frontiers of Quantum Chromodynamics (QCD) and, in a wider sense, the "Standard Model of Elementary Interactions". The data resulting from the CERN SPS, BNL AGS and GSI SIS experiments, and in particular also from almost a decade of experiments carried out at the "Relativistic Heavy Ion Collider"(RHIC) at Brookhaven, have been fully analyzed, uncovering a wealth of information about both the confined and deconfined phases of QCD at high energy density.