Fluctuation Solution Theory connects certain thermodynamic partial derivatives to integrals of molecular correlation functions from statistical mechanics. This paper describes some applications of this method for correlating and predicting properties of solutions of chemical engineering interest. The method has been used for three kinds of practical systems : A) the densities and compressibilities of liquids; B) the densities and phase equilibria of supercritical substances with liquids; and C) the densities and activities of aqueous electrolytes. The paper illustrates the method for these cases and gives recent results.