SOLUBILITY PARAMETERS ARE USED IN THE COATings industry to select solvents. Liquids with similar solubility parameters will be miscible, and polymers will dissolve in solvents whose solubility parameters are not too different from their own. The basic principle is “like dissolves like.” Solubility parameters help put numbers into this simple qualitative idea. The solubility parameter approach has been used for many years to select solvents for coating materials. The lack of total success has stimulated research. The skill with which solvents can be optimally selected with respect to cost, solvency, workplace environment, external environment, evaporation rate, flash point, etc., has improved over the years as a result of a series of improvements in the solubility parameter concept and widespread use of computer techniques. Most, if not all, commercial suppliers of solvents have computer programs to help with solvent selection. One can now easily predict how to dissolve a given polymer in a mixture of two solvents, neither of which can dissolve the polymer by itself. This contribution to the paint testing manual unfortunately cannot include discussion of all of the significant efforts leading to our present state of knowledge of the solubility parameter. An attempt is made to outline developments, provide some background for a basic understanding, and give examples of uses in practice. The key is to determine which affinities the important components in a system have for each other. For coatings, this means affinities of solvents, polymers, pigment surfaces, additives, and substrates.