One of the most powerful and pervasive influences on consumer behavior is variously described as “social communication,” “word-of-mouth,” “opinion leadership,” or “buzz.” These terms all refer to the effects that consumers have on each other when they communicate. The importance of off-line social communication has long been recognized by social scientists (e.g., Lazarsfeld, Berelson, & Gaudet, 1944) and especially by marketing management (Dichter, 1966; Whyte, 1954). Good word-of-mouth (WOM), the term most often used in business, is still considered to be the most effective form of promotion, so it is highly valued by marketers (e.g., Dye, 2000; Walker, 1995). The advent of the Internet and the growth of the World Wide Web, however, have given consumers an entirely new realm in which they can communicate and thus influence each other (Negroponte & Maes, 1996). That they do so with a vengeance is evidenced by the sheer amount of social communication online, by the many forms these interactions assume, and by the grudging acknowledgment by marketers and managers that this has become a vital component of e-commerce (e.g., Kirkpatrick & Roth, 2005). This article explains some of the theoretical aspects of social influence, describes the many ways social influence operates online, and suggests methods by which marketers can manage this force to benefit their brands.