A macroscopic system consists of a tremendous number of microscopic atoms and molecules. In thermal equilibrium the state of such a system is uniquely defined, despite the fact that the microscopic particles behave quite randomly. This observation gives rise to the fundamental law of the statistical physics; it allows entropy to be defined and a framework for the theory to be constructed but cannot be derived form quantum mechanics or force laws. Introduction to Statistical Physics seeks to explain the laws of the macroscopic level to undergraduate students learning them for the first time. The first part of this book explains the essence of statistical physics without going into details such as Liouville’s theorem or ergodic theorem, which are difficult for beginners and unnecessary for actual application of the statistical mechanics. In the second part, statistical mechanics ís applied to various systems which look different but have the same mathematical structure, in particular, features applications to quantum dynamics, thermodynamics, Ising model and statistical dynamics of free spins. Advanced topics in phase transitions and dense gases conclude the text, plus helpful appendices. A macroscopic system consists of a tremendous number of microscopic atoms and molecules. In thermal equilibrium the state of such a system is uniquely defined, despite the fact that the microscopic particles behave quite randomly. This observation gives rise to the fundamental law of the statistical physics; it allows entropy to be defined and a framework for the theory to be constructed. The first part of this book explains the essence of statistical physics without going into details such as Liouville's theorem or ergodic theorem, which are difficult for beginners and unnecessary for actual application of the statistical mechanics. In the second part, statistical mechanics ís applied to various systems which look different but have the same mathematical structure. In this way readers can deepen their understanding of statistical physics. This book provides a comprehensive presentation of the basics of statistical physics. The first part explains the essence of statistical physics and how it provides a bridge between microscopic and macroscopic phenomena, allowing one to derive quantities such as entropy. Here the author avoids going into details such as Liouvilles theorem or the ergodic theorem, which are difficult for beginners and unnecessary for the actual application of the statistical mechanics. In the second part, statistical mechanics is applied to various systems which, although they look different, share the same mathematical structure. In this way readers can deepen their understanding of statistical physics. The book also features applications to quantum dynamics, thermodynamics, the Ising model and the statistical dynamics of free spins. A macroscopic system consists of a tremendous number of microscopic atoms and molecules. In thermal equilibrium the state of such a system is uniquely defined, despite the fact that the microscopic particles behave quite randomly. This observation gives rise to the fundamental law of the statistical physics; it allows entropy to be defined and a framework for the theory to be constructed. The first part of this book explains the essence of statistical physics without going into details. In the second part, statistical mechanics is applied to various systems which look different but have the same mathematical structure This reference introduces the fundamental law of statistical physics, defines entropy, and constructs the framework of the theory. The statistical physics thus constructed is applied for various concrete systems