Statistical physics bridges the gap between macroscopic and microscopic worlds, which originated in attempts to explain how the irreversible macroscopic behaviors can be reconciled with the reversible microscopic Hamiltonian equations of motion, in other words, why the entropy increases and how the direction of time arrow becomes certain. Statistical physics aims at a fundamental understanding of how the macroscopic collective properties emerge from interactions of many degrees of freedom, which cannot be simply deduced from an understanding of individuals. Representative examples of the emergent phenomena are phase transitions and criticalities, associated with singularities of thermodynamic functions, which signals the existence of universal physical mechanism underlying seemingly quite distinct systems. Contrary to other disciplines of physics, the scope of statistical physics is not restricted to specific systems but is rather unlimited and has found applications in broad areas, with considerable success, encompassing from solid-liquid-gas transitions of classical systems to macroscopic quantum effects such as superconductivity and superfluidity.