The basic objective in this book is to determine the dynamical role of the lattice structure for structural changes. In this chapter, thermodynamic principles are reviewed for crystalline states, realizing that the concept of order variables should be redefined in relation to lattice excitations. Although implicit in thermodynamic functions, the lattice is dynamically essential for structural and superconducting transitions in crystals. It is also significant that order variables emerge in finite amplitude at a critical temperature as related with an adiabatic potential in a crystal. Here, we introduce necessary modifications of the traditional principles, serving as a preliminary account of thermodynamics of crystalline states.