The aim of this chapter is to provide a general review of research published in the core terrorism studies journals between 1990 and 2007. As such, this chapter seeks to build on a long history of past reviews and assessments of the literature (Reid, 1983; Schmid and Jongman, 1988; Merari, 1991; Silke, 2004c) which have attempted at various points to assess the state of the art on this subject. Terrorism has never been an easy subject to study or research. In particular, relentless (and ongoing) conceptual and definitional disagreements, a shortage of experienced researchers, and a general lack of funding for studies have all presented formidable obstacles. Indeed, in the 1990s, there were probably only about 100 researchers across the world researching and writing on terrorism on a regular basis (Silke, 2004c). One consequence of such an environment is that studies which are expensive in terms of funds or time have been scarce as the resources have not been present to carry them out. This has hindered the progression of the field and much work is simply rehashing old data or providing primarily descriptive or exploratory analysis.