This chapter explores open source databases on terrorism. These are created from unclassified, publicly available information retrieved from both print and digital media. The unit of analysis for these databases are events, organizations, or individuals. The advantage of event- and group-level databases is that they are worldwide in scope. In contrast, individual-level databases are more focused on perpetrators in a single country. However, open-source databases can be susceptible to media inaccuracies and government censorship. There is also the issue of a lack of systematic empirical validation. The chapter notes possible improvements that can be made to open-source databases such as better coverage of domestic terrorism, automated coding, use of geo-spatial information, and more detailed data on the effectiveness of countermeasures.