Marketers have always been aware of the effect of word of mouth, and there is clearly an art to effective word-of-mouth campaigning. Yet the science behind word-of-mouth equity helps reveal how to hone and deploy that art: it shows which messages consumers are likely to pass on and the impact of those messages, allowing marketers to estimate the tangible effect word of mouth has on brand equity and sales. These insights are essential for companies that want to harness the potential of word of mouth and to realize higher returns on their marketing investments.