Application of regular solution thermodynamic theory to polymer solutions began with the Flory-Huggin's equation in 1942. The major contributions since then have been the Prigogine theory, 1957, and the Heil-Prausnitz theory, 1966. No theory has yet predicted quantitatively the dependence of miscibility on (1) temperature and (2) concentration over extensive ranges for nonvolatile solvents (plasticizers). Progress is being made, however, and this review attempts to summarize this progress.