The sub series Ternary Alloy Systems of the Landolt Börnstein New Series provides reliable and comprehensive descriptions of the materials constitution, basedo ncritical intellectual evaluationso fall data available at the time and it critically weights the different findings, also with respect to their compatibility with today’s edge binary phase diagrams. Selected are ternary systems of importance to alloy development and systems which gained in the recent years otherwise scie ntific interest. In one ternary materials system, however, one may find alloys for various applications , depending on the chosen composition. Reliable phase diagrams provide scientists and engineers with basic information of eminent importance for fundamental research and for the developmentand optimization of materials. So collections of such diagrams are extremely useful, if the data on which they are based have been subjected to critical evaluation, like in these volumes. Critical evaluation means: there where contradictory information is published data and conc lusions are being analyzed, broken down to the firm facts and re interpreted in the light of all present knowledge. Depending on the information available this can be a very difficult task to achieve. Criticaleval uations establish descripti ons of reliably known phase configurations and related data.