Support for this research was provided by the Department of Homeland Security (DHS) through the National Consortium for the Study of Terrorism and Responses to Terrorism (START), grant N00140510629, and from the National Institute of Justice (NIJ), grant 2005-IJ-CX-0002. Any opinions, findings, and conclusions or recommendations in this document are those of the authors and do not necessarily reflect views of the DHS or NIJ. Some of the materials in this essay are revisions to a final report for the NIJ grant, coauthored with Kim Cragin and Anna Kasupski. We thank David Cook, James Hendrickson, Erin Miller, and Amber Stoesser for their assistance. We would also like to thank our colleagues in the START Consortium for their help and support. Although social science research on terrorism has expanded dramatically, especially since the 1970s, 1 the number of studies based on systematic empirical analysis is surprisingly limited. In an encyclopedic review of political terrorism, Schmid and Jongman (1988) identify more than 6,000 published works but note that "there are probably few areas in the social science literature in which so much is written on the basis of so little research" (p. 179). More recently, Lum, Kennedy, and Sherley (2006) reviewed more than 20,000 articles on terrorism published between 1971 and 2004 and found only seven that met their criteria of being moderately rigorous evaluation studies.