The two-fluid model introduced in Chap. 6 is limited to stiffened-gas equations of states. Although simple, the stiffened-gas equation of state has a limited range of validity [1]. Models supporting more general equations of states are thus needed. Moreover, the two-fluid model in Chap. 6 does not support capturing phase change. To construct and understand models supporting higher flexibility in the choice of equations of states and the type of interactions among phases, we need to discuss the fundamental thermodynamics property relations, equilibrium conditions, and derivation of thermodynamics properties from experimentally obtained general equations of states. The computation of such complex models also relies on knowing equations for properties such as internal energy, entropy, chemical (or Gibbs) potential, and sound speed for a general equation of state.