Weak ties are relationships between individuals marked by relatively low levels of intensity and emotional closeness. By contrast, strong ties are relationships that involve high levels of intensity and emotional closeness. The importance of weak ties to a variety of sociological phenomena has been most influentially articulated by Mark Granovetter in the most cited article in sociology, “The Strength of Weak Ties” (SWT). Granovetter argues that most people intuitively expect strong ties to generally be more important than weak ties, because those to whom we are closely tied are more motivated to help us, and are also more likely to be stronger sources of social influence and social support. However, basing his argument on principles of social psychology, Granovetter argues that weak ties are – paradoxically – more important for a variety of phenomena, from helping people obtain a job, to the diffusion of ideas and innovations, to facilitating collective action. This entry describes the key theoretical underpinnings of Granovetter's SWT argument, and its application to various topics including job searches, and collective action and social movements. Also discussed are contextual issues, conceptualization and measurement, virtual networks, and related problems and future research.