This chapter explains heat capacity, heat content, and energy storage. Heat capacity, which is the most fundamental of all thermal properties, is related to the strength of intermolecular interactions, phase stability, thermal conductivity, and energy storage capacity. The thermal properties of any material are among the most fundamental, whether directly or indirectly involved in the material's application. The chapter explores the fundamentals of thermal properties, from basic thermodynamics to applications. The heat capacity of the rigid glass is still in excess of that of the crystalline solid, and this can be understood in terms of Debye's heat capacity theory applied to each phase. The fact that the heat capacity of the glass is higher than that of the crystal indicates that the effective Debye characteristic temperature of the glass is lower than that of the crystal. Although they are at the same initial temperature, the toast cools faster because it has a lower heat capacity.