The rapidly evolving social media platforms, with an estimated 1.9 billion users worldwide, offer a myriad of communication benefits and risks in the context of a disaster. Social media generally refers to internet-based technologies that enable people to interact and share resources and information using either text or multimedia applications (Lindsay 2011; Dabner 2011). Advances in mobile devices allow access to anyone who has the ability to connect online (Abbasi et al. 2012). For example, the microblogging platform Twitter allows followers to track what an account holder is doing and thinking in real time within the confines of 140 characters (Kaigo 2012). Tweets can be sent from a variety of platforms ranging from cell phones to computers. Other examples of social media platforms today are Facebook, YouTube, Qzone, Pinterest, Instagram, and Flickr. This entry describes how social media, in particular Twitter, can be used for a variety of applications before, during, and after an earthquake, both inside and outside the area of impact. These include detection, warnings, connecting to survivors, situational awareness, notifying responders of where help is needed, and galvanizing humanitarian aid. Importantly, the increasing participation of “citizen seismologists” via social media is filling the information gap with field observations immediately after an earthquake (Young et al. 2013) for both first responders and survivors. In fact, social media, in particular Twitter, may be the only immediate source of data from locations with limited sensors or other scientific instruments. Obviously, not all earthquakes are reported on social media as many events are in remote areas or undersea and in countries with limited social media access or the magnitude is too small to be felt. A unique benefit of social media is that it is user generated – disaster agencies, seismologists, and other parties do not have to motivate citizens to tweet – they will do it anyway, potentially by the thousands in a significant earthquake. The challenge is how to transform the rapidly spreading flood of real-time information, some of it inaccurate, into reliable, useful, and valuable data. Part of the solution is to train Twitter users to tweet messages that can be more easily analyzed both manually and automatically using a crisis-specific syntax (Starbird and Stamberger 2010). This approach, among others discussed in this entry, will help guide disaster response, galvanize ongoing humanitarian efforts, and add value to the expanding body of earthquake sciences gathered since the development of the modern seismograph in the late 1800s.