For many years there has been controversy about the value of pass-on readers. Some media buyers prefer circulation as their criterion, most feel that publications with very high numbers of readers-per-copy cannot be judged by readership alone in the same way as other publications. This review of reports from 1963 to 1978 is a valuable summary. The case that primary readers are more valuable is well made. The actual weight to give them is still a matter of opinion. But this paper should help us to make up our own minds.