Utilities and regulators need to know what, if any, effect time-of-use (TOU) rates will have on electricity loads. The question raises serious methodological challenges because no means of measurement is perfect. The authors explore these difficulties and then apply alternative measures to actual load data drawn from European and United States utility systems. They find wide variability in responsiveness to TOU rates from one industry to another and between utility systems, depending on the method used and the length of time rates have been in effect. They conclude that TOU rates clearly change load shapes but are uncertain whether United States utilities will ever see the degree of response found in European data. Published in [Public] [Utilities Fortnightly], May 22, 1980, following several articles that have taken exception to earlier work by RAND authors on this subject.