Use of residence time distributions (RTD's) for the design of clarifiers proposed by the senior author in 1969 and by others has been found to be based on flow pattern assumptions which are false. Since the residence time distribution depends upon the density of the feed to the clarifier, flow rate and other near‐intangibles, it is not possible to obtain RTD's which pertain to an operating settler from which reliable performance may be predicted. Finally, laboratory studies demonstrate that the combination of RTD's and quiescent settling data do not reliably predict clarifier performance.