Opinion leadership is leadership by an active media user and who interprets the meaning of media messages or content for lower-end media users. Typically the opinion leader is held in high esteem by those who accept his or her opinions. Opinion leadership comes from the theory of two-step flow of communication propounded by Paul Lazarsfeld and Elihu Katz Significant developers of the theory have been Robert K. Merton, C. Wright Mills and Bernard Berelson. This theory is one of several models that try to explain the diffusion of innovations, ideas, or commercial products.
Merton distinguishes two types of opinion leadership: monomorphic and polymorphic. Typically, opinion leadership is viewed as a monomorphic, domain-specific measure of individual differences, that is, a person that is an opinion leader in one field may be a follower in another field. An example of a monomorphic opinion leader in the field of computer technology, might be a neighborhood computer service technician. The technician has access to far more information on this topic than the average consumer and has the requisite background to understand the information, though the same person might be a follower at another field (for example sports) and ask others for advice. In contrast, polymorphic opinion leaders are able to influence others in a broad range of domains. Variants of polymorphic opinion leadership include market mavenism, personality strength  and generalized opinion leadership. So far, there is little consensus as to the degree these concept operationalize the same or simply related constructs.
In his article "The Two Step Flow of Communication" by Elihu Katz, he found opinion leaders to have more influence on people's opinions, actions, and behaviors than the media. Opinion leaders are seen to have more influence than the media for a number of reasons. Opinion leaders are seen as trustworthy and non-purposive. People do not feel they are being tricked into thinking a certain way about something from someone they know. However, the media can be seen as forcing a concept on the public and therefore less influential. While the media can act as a reinforcing agent, opinion leaders have a more changing or determining role in an individual’s opinion or action.
Opinion leaders are individuals who obtain more media coverage than others and are especially educated on a certain issue. They seek the acceptance of others and are especially motivated to enhance their social status. In the jargon of public relations, they are called thought leaders.
In a strategic attempt to engage the public in environmental issues and his nonprofit, The Climate Project, Al Gore used the concept of opinion leaders. Gore found opinion leaders by recruiting individuals who were educated on environmental issues and saw themselves as influential in their community and amongst their friends and family. From there, he trained the opinion leaders on the information he wanted them to spread and enabled them to influence their communities. By using opinion leaders, Gore was able to educate and influence many Americans to take notice of climate change and change their actions.
- .Katz, E.; Lazarsfeld, P. F. (1957). Personal influence (E. ed.). New York: Free Press. More than one of
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- Merton, R. K. (1957). Social theory and social structure. Glencoe: Free Press.
- Childers, T. L. (1986). "Assessment of the psychometric properties of an opinion leadership scale". Journal of Marketing Research 23: 184–188. doi:10.2307/3172527.
- Flynn, L. R.; Goldsmith, R. E.; Eastman, J. K. (1996). "Opinion leadership and opinion seekers: Two new measurement scales". Journal of the Academy of Marketing Science 24: 147–147. doi:10.1177/0092070396242004.
- Feick, L. F.; Price, L. L. (1987). "The market maven: A diffuser of marketplace information". Journal of Marketing 51: 83–97. doi:10.2307/1251146.
- Weimann, G. (1991). "The influentials: Back to the concept of opinion leaders?". Public OpinionQuarterly 55: 267–279. doi:10.1086/269257.
- Gnambs, T.; Batinic, B. (2011). "Evaluation of measurement precision with Rasch-type models: The case of the short Generalized Opinion leadership Scale". Personality and Individual Differences 50: 53–58. doi:10.1016/j.paid.2010.08.021.
- Gnambs, T.; Batinic, B. (2011). "Convergent and discriminant validity of opinion leadership: Multitrait-multimethod analysis across measurement occasion and informant type". Journal of Individual Differences 39: 94–102. doi:10.1027/1614-0001/a000040.
- Katz, Elihu (1957). "The two-step flow of communication: An up-to-date report on an hypothesis". Public Opinion Quarterly 21: 61–78. doi:10.1086/266687.
- Rose, P.; Kim, J. (2011). "Self-Monitoring, Opinion Leadership and Opinion Seeking: a Sociomotivational Approach". Current Psychology 30: 203–214. doi:10.1007/s12144-011-9114-1.
- Eric Johnson, SVP - General Manager of InsiteResearch - An Advanced Health Media Company "Hitting the Mark with KOL Management: Identification, Segmentation and Targeting" http://www.insiteresearch.net/LinkClick.aspx?fileticket=N7jFNFTGDbw=&tabid=87
- Matthew Nisbet, Ph.D, Professor at American University, "A Two Step Flow of Influence?: Opinion Leader Campaigns on Climate Change" Science Communication, March 2009, 30.
- Terry Nugent, Director of Marketing, Medical Marketing Service, Inc. (MMS)
- "Developing Win-Win Key Opinion Leader Relationships," Pharma Marketing News, Vol. 2, #10; Reprint #210-01. http://www.pharma-mkting.com/news/pmn210-article01.html
Further reading 
- Bodendorf, F., Kaiser, C. (2009). "Detecting opinion leaders and trends in online social networks". Proceeding of the 2nd ACM workshop on Social web search and mining.
- Katz, E., Lazarsfeld, P. F. (1955)." Personal influence, the part played by people in the flow of mass communication", Glencoe, Free Press.
- Keller E. B., Berry J.(2003). "The influentials", New York, Free Press.
- Song, X., Chi, Y., Hino, K., Tseng, B. (2007). Identifying opinion leaders in the blogosphere. Proceedings of the sixteenth ACM conference on Conference on information and knowledge management
- Valente, T. (2007). "Identifying Opinion Leaders to Promote Behavior Change". Health Education Behavior 34(6), pp. 881–896
- Weimann, W. (1991). "The Influentials: Back to the Concept of Opinion Leaders". Public Opinion Quarterly 55(2),pp. 267–279.