This paper reviews the empirical evidence of the effect of credibility of the message source on persuasion over a span of 5 decades, primarily to come up with recommendations for practitioners as to when to use a high- or a low-credibility source and secondarily to identify areas for future research. The main effect studies of source credibility on persuasion seem to indicate the superiority of a high-credibility source over a low-credibility one. Interaction effect studies, however, show source credibility to be a liability under certain conditions. The variables found to interact with source credibility are categorized into 5 categories: source, message, channel, receiver, and destination variables. The most heavily researched variables have been the message and receiver variables. Implications for marketers/advertisers and suggestions for future research are discussed.