The pharmaceutical industry uses some physicians, called key opinion leaders or KOLs, as intermediaries to influence prescribing physicians. One marketing firm says, for example, that “KOL” is “a convenient shorthand for those people – usually eminent, usually physicians – who we co-opt into our development and marketing strategies.” I describe the recruitment and deployment of KOLs, as presented by marketers and others. The KOL concept is owed in part to the work of sociologist Paul Lazarsfeld and his students, studying the influence of mass media on voting behaviour. In this presentation, I introduce that history in order to show how the pharmaceutical industry has successfully established a structure of influence it can use.