Thermodynamic data are required for an understanding of the behavior of materials hut are often lacking (or even unreliable) for a variety of reasons such as synthetic problems, purity issues, failure to correctly identify hydrolysis products, instability of the material. etc. Thus, it is necessary to develop procedures for the estimation of that data. The Thermodynamic Difference Rules (TDR) are additive approximations by which the properties of materials are estimated by reference to those of related materials. These rules appear in the form of the reliable Hydrate Difference Rule (HDR), based on the well-established properties of the large number of known hydrates, and the somewhat less certain Solvate Difference Rule (SDR). These rules are briefly surveyed and their application carefully delineated by a scheme and demonstrated by a number of calculated examples.