The problem of defining a useful and quantitative measure of the performance of a photo-optical system has engaged the attention of many investigators for the last twenty years or more. The results have not so far been encouraging. The problem is twofold: first, to achieve a means for measuring the output material in an effective quantitative manner and, second, to interpret the results into a measure of real information content. Modern developments in modulation transfer analysis have helped toward achieving success in the first part; it is in the second half of the problem that much work remains to be done. This paper reviews the application of transfer functions to performance measure-ment and prediction, presents the results of some experimental work which has been done, and suggests some possible approaches to the solution of the remaining problems, with the intention of simulating more work in this area.