This book presents the fundamental principles of Classical thermodynamics, supplemented by an overview of Statistical Thermodynamics, and explores their applications to a wide variety of problems in the Earth and Planetary sciences, spanning the range from the Earth's core, mantle and crust, aqueous solutions and fluid-rock interactions to solar nebula, terrestrial planets and asteroids. The topics covered include the laws of classical thermodynamics, microscopic foundations of thermodynamic properties, critical phenomena and near-critical processes, equations of state, phase transitions and phase equilibria, the Earth's interior and adiabatic processes, terrestrial magma ocean, thermodynamics of solutions, element and stable isotope fractionations, electrolyte solutions and electrochemistry, osmotic processes, surface effects, entropy production and kinetic implications, thermodynamics of chemical diffusion, estimation of thermochemical properties, and mathematical methods of classical thermodynamics. There is also a wealth of solved problems relating to both fundamental concepts and applications to natural processes.