Strong credibility is a marketplace advantage for consultants, but factors that contribute to credibility are not well understood. Literature on reputation, competency, and effectiveness is summarized. Research on interpersonal, personal, and experiential factors is reviewed. A common theme in the literature and research is emphasis of relationship skills and deemphasis of personality traits, technical competence, and credentials, which is in contrast with typical consultant training programs and day-to-day management of consulting practices. Recommendations for consultant training and further research are to concentrate on relationship variables and transactions in the context of consulting roles.