The prediction of wet runway friction has been studied intensively over the years by several researchers. Previous attempts to predict airplane performance based on friction vehicle data have had limited success. Past research has shown that extensive, controlled testing of a specific airplane and a suitable ground vehicle is required over a broad operational spectrum to develop prediction equations that are unique to that aircraft. The NASA Hydroplaning Theory and the limited test data and assumptions on which that concept is based is discussed. A review is presented of the past data, the reasons for lack of correlation, the inadequacies of recent attempts at correlation using the hydroplaning theory, and the possible uses of friction vehicles to provide meaningful information to the airline crews.