Thermodynamics is a practical science and also deals with these inhomogeneous dynamic systems provided the thermodynamical parameters are well-defined. The present article takes a gradual approach to the subject, starting with a focus on cyclic processes and thermodynamic equilibrium, and then gradually beginning to further consider non-equilibrium systems. For thermodynamics and statistical thermodynamics to apply to a process in a body, it is necessary that the atomic mechanisms of the process fall into just two classes: those so rapid that, in the time frame of the process of interest, the atomic states effectively visit all of their accessible range, and those so slow that their effects can be neglected in the time frame of the process of interest. The book will be of use to the students, researchers and general readers of this subject.