This chapter is an outline of the thermodynamic basis for modeling crystal-melt equilibria. Because the purpose of the chapter is illustrative, rather than rigorous applications to specific problems, numerical analysis can he kept to a minimum by treating solutions as ideal, thus eliminating the effects of activity coefficients. In addition, the standard state Gibbs energies of crystallization are assumed to be linear functions of temperature and pressure. This eliminates the numerical problems that arise from thermal expansion, compressibility and heat capacity changes. Once the features of ideal systems are mastered, the principles involved in modeling real systems are easily followed.