This article addresses the use of the Global Terrorism Database (GTD) for researchers of terrorism. This open source database provides information on domestic and international terrorist attacks. The author examines how data are selected and categorized in the GTD, drawing attention to changing methodological practices since the 1970s. The article, using the GTD, highlights the geographic distribution and intensity of terrorist attacks, as well as the sources and tactics of terrorism. The author also considers the weaknesses of open source databases while noting the growing potential of these tools in providing meaningful insights into terrorist activities.