This textbook outlines the fundamentals required to understand the science of thermodynamics and describes the application of thermodynamics in geochemistry. The work is distinguished by its comprehensive, balanced coverage and its rigorous presentation of geological thermodynamics and geochemistry. The authors bring years of teaching experience to the work, and have attempted to particularly address those areas where other texts on the subject have provided inadequate coverage. A thorough review of the necessary mathematics is presented early on, both as a refresher for those with a background in university calculus, and for the benefit of those coming to the subject for the first time. The text is written for students in advanced undergraduate or graduate level geochemistry as well as for all researchers in this field.