This paper, part of the series of papers resulting from Harvard’s Executive Session on Policing and Public Safety, examines the use of social media by police departments in the aftermath of the 2013 bombing at the Boston Marathon. The paper describes the Boston Police Department’s (BPD’s) response to the bombing and how its use of various types of social media enhanced the investigation, kept the public informed, and brought together various law enforcement agencies to identify and apprehend the suspects. The department made extensive use of its Twitter account and its Facebook page to post information about the suspects and correct any misinformation that had been given to the media. The paper discusses the lessons that can be learned from the BPD’s use of social media during a crisis, and how some of the most important characteristics of social media are compatible with the best traditions of policing. These characteristics are 1) the scope of social media and its ability to connect with a significant portion of the public; 2) the structure of social media which enables the police to have a two-way conversation with the community; and 3) the tone of social media – informal, conversational, sometimes humorous, and distinct from traditional press releases and marketing messages. The paper also addresses the speed of social media and its ability to relay important information to the public, as well as the challenges faced by police departments when using social media. These challenges include monitoring and managing public information, tone, internal coordination and control, and measuring what matters.