This manual has been prepared by Subcommittee IV of ASTM Committee E-20 on Temperature Measurement. The responsibilities of ASTM Committee E-20 include “Assembling a consolidated source book covering all aspects relating to accuracy, application, and usefulness of thermometric methods.” This manual is addressed to the thermocouple portion of this responsibility. The contents include principles, circuits, standard electromotive force (emf) tables, stability and compatibility data, installation techniques, and other information required to aid both the beginner and the experienced user of thermocouples. While the manual is intended to be comprehensive, the material, however, will not be adequate to solve all the individual problems associated with many applications. To further aid the user in such instances, there are numerous references and an extensive bibliography. In addition to presenting technical information, an attempt is made to properly orient a potential user of thermocouples. Thus, it is hoped that the reader of this manual will make fewer mistakes than the nonreader. Regardless of how many facts are presented herein and regardless of the percentage retained, all will be for naught unless one simple important fact is kept firmly in mind. The thermocouple reports only what it “feels.” This may or may not be the temperature of interest. The thermocouple is influenced by its entire environment, and it will tend to attain thermal equilibrium with this environment, not merely part of it. Thus, the environment of each thermocouple installation should be considered unique until proven otherwise. Unless this is done, the designer will likely overlook some unusual, unexpected, influence.