This book provides detailed information on the history, analysis and applications of chlorine and bromine isotope geochemistry. This largely extended 2nd edition has incorporated most research published between 2014 and 2023, and contains also significantly more information on the earliest experimental data obtained during the early 20th century. This 2nd edition describes isotope studies done on terrestrial samples, and also incorporates the fascinating chlorine (and bromine) isotope ratio investigations obtained from extraterrestrial and astronomical studies, and thus it shows the full extent of the current status of knowledge of this subject.