Received December 27, 1976 Accepted for Publication September 7, 1977 A review of the characteristics of available fuel rod reliability models reveals an extremely wide range of opinion regarding the scale of complexity appropriate to the problem. It is argued that this diversity of opinion is symptomatic of a model building style in which no attention is formally paid to the uncertainty in the model predictions. An information-theory-based methodology is suggested as a means for systematically building a model in which the information content of the prediction is no more and no less than the information content of the supporting data.