This chapter describes the basics of energy and entropy and “free energy.” Fundamentals of internal energy U, the enthalpy H, entropy S, free energies G, and F of a substance are presented. The chapter also presents the thermal vibration model to promote a better understanding of the U, S, and F of the crystal. It covers basic concepts of thermodynamics of magnetic transition and discusses the role and the meaning of magnetic transition in iron and steel. The chapter concludes with a general discussion on an amorphous phase from a thermodynamic viewpoint.