Twenty-five years of research on predicting properties are reviewed on the basis of projects resulting in the ongoing American Petroleum Institute’s Technical Data Book - Petroleum Refining and the AIChE-DIPPR’s Data Prediction Manual and Data Compilation. While methods at this time are accurate for some properties and certain classes of compounds and their mixtures, very little improvement is evident for other properties. While many weaknesses are directly attributable to a paucity of data available for deriving prediction methods, complexity of certain molecules and their interactions have yet defied development of useful prediction techniques. Properties to be discussed will be limited to liquids and vapors and will include basic thermophysical properties, derived thermal and equilibrium properties, and transport properties. Results from evaluation of predictive methods against experimental data will be used for assessment of the strengths and weaknesses of current methods. Suggestions for data necessary for development of improved predictors will be made. Properties to be discussed include density including PVT relations, vapor pressure fi om the triple to the critical point, critical properties, enthalpies and entropies of formation and phase transition, thermal properties, viscosity, thermal conductivity, diffusivity, and surface tension.