This chapter examines the concept of ethos, or source credibility, and its relationship to persuasion. It also examines credibility as it relates to celebrity endorsers and spokespersons. The chapter offers a definition, discusses the basic features, and explores the underlying dimensions that make up credibility. It considers how credibility functions according to Petty and Cacioppo's elaboration likelihood model of persuasion, and it examines a phenomenon known as the sleeper effect. The chapter discusses credibility as it applies to both institutions and interpersonal settings. An important feature of O'Keefe, D. J. definition is the recognition that credibility is a receiver-based construct. Credibility exists in the eye of the beholder. Just as chefs are interested in what ingredients go into award-winning recipes, persuasion researchers have tried to determine the "ingredients," or underlying dimensions, of credibility.