First and Second Laws -- Thermodynamic Potentials and Derivative Properties -- Third Law and Thermochemistry -- Critical Phenomenon and Equations of States -- Phase Transitions, Melting and Reactions of Stoichiometric Phases -- Thermal Pressure, Earth's Interior and Adiabatic Processes -- Thermodynamics of Solutions -- Thermodynamic Solution Models: Non-electrolytes -- Equilibria Involving Solutions and Gaseous Mixtures -- Element Fractionation in Geological Systems -- Electrolyte Solutions and Electrochemistry -- Surface Effects. When the knowledge is weak and the situation is complicated, thermodynamic relations are really the most powerful Richard Feynman Thermodynamicshas playeda major role in improvingour understandingof natural processes, and would continue to do so for the foreseeable future. In fact, a course in thermodynamics has now become a part of Geosciences curriculum in many - stitutions despite the fact that a formal thermodynamics course is taught in every other department of physical sciences, and also in departments of Chemical En- neering,MaterialsSciencesand BiologicalSciences. The reasonthermodynamicsis taught in a variety of departments, probably more so than any other subject, is that its principles have wide ranging applications but the teaching of thermodynamics also needs special focus depending on the problems in a particular ?eld. There are numerous books in thermodynamics that have usually been written with particular focus to the problems in the traditional ?elds of Chemistry, Physics and Engineering. In recent years several books have also been written that emp- sized applications to Geological problems. Thus, one may wonder why there is yet another book in thermodynamics. The primary focus of the books that have been written with Geosciences audience in mind has been chemical thermodynamics or Geohemical thermodynamics. Along with expositions of fundamental principles of thermodynamics, I have tried to address a wide range of problems relating to g- chemistry,petrology,mineralogy,geophysicsand planetarysciences. It is not a fully comprehensive effort, but is a major attempt to develop a core material that should beofinteresttopeoplewith differentspecialties intheEarthandPlanetarySciences.