Despite a heavy barrage of advertising, most consumers declare that their purchases are most influenced by the experiences, advice, and recommendations of others, and not by marketers. Interpersonal communication between and among consumers serves as a potent path for influence, and by 2022, marketing managers are expected to spend $15 billion on influencer marketing, a marketing technique in which companies partner with people with specialized knowledge, expertise, authority, social position, and/or personal relationships that enable them to have influence over others to co-produce marketing messages to promote their brands via offline and electronic word-of-mouth. This technical note provides an overview of influencer marketing and discusses the various roles that influencers play for marketers, including delivering marketing messages in a more authentic voice and setting than that offered by more traditional advertising media where messages are authored by firms. It categorizes different types of influencers, explains the social psychological processes that make interpersonal influence work, and provides guidelines on how to identify people with influence. It discusses the opportunities and challenges related to measuring the performance of influencers and assessing the costs and return-on-investment of influencer marketing programs. Finally, it highlights emerging risks in the contemporary influencer marketing landscape.