Word of mouth (WOM) reflects informal communication between private individuals that evaluates goods and services (Anderson, 1998). It provides a highly credible means of persuasion because the communicator is not seen as having a vested interest in selling the recommended product or service. In this chapter, WOM is conceptualized as a type of advice between private parties (typically consumers), focused on goods or services, and not necessarily purposeful or directive. WOM is a very powerful tool, especially given the relatively new focus in marketing on customer engagement and the customer’s role in value co-creation in the context of market offerings. However, over the decades, a number of myths have developed surrounding the effects of WOM. These myths are challenged within a review of the research literature. Suggestions for future research directions, research methods to capture WOM, and some best practices are also discussed.