Because of its interdisciplinary nature, it remains a point of debate whether there is one field of terrorism studies or several. But regardless of the particular perspective from which this subject is approached, two methodological concerns are quick to draw attention. The first is the oft-repeated problem that a commonly accepted definition of ‘terrorism’ does not (yet) exist. This raises some fundamental concerns about the study of terrorism, not the least of which is how to study what cannot be accurately defined. Another issue to plague the study of terrorism, has been a persistent lack of research based on primary sources. Although discussions on the state of the art in terrorism studies have frequently given the definition-issue central stage, the over-reliance on secondary-sources based research is arguably no less deserving of our attention.

Primary sources provide first-hand insights into, or descriptions of, a certain subject. Their close proximity to the subject under investigation makes them particularly valuable. Secondary sources are farther removed from that subject. They offer others’ interpretations or analyses of a particular event or artifact, which brings with it potential issues of factual accuracy and impartiality. Some examples of primary sources in terrorism studies are interviews with (former) terrorists, raw data derived from police investigations into terrorism-related activities or information obtained through direct observation. There are very good reasons why such sources are rare in studies on terrorism. Terrorists, even former ones, can be hard to find and harder to convince to cooperate in interviews. Government agencies with access to information on terrorism are generally very reluctant to share such sensitive information. Participant observation runs into both practical and ethical obstacles.

While recent years have seen an increase in primary-sources based studies and related research methodologies, such publications remain a minority. The majority of the vast literature on terrorism refers to other such books and articles to support the claims being presented. In turn, these articles and books refer to yet another set of secondary sources and so forth. The scarcity of primary-sources based research poses a challenge to the empirical validity of the explanations presented, and leads to little new information making into the field. These issues, their causes and the ways in which they are being addressed, are discussed in detail in the ICCT publication, ‘Moving Terrorism Research Forward’ by Bart Schuurman and Quirine Eijkman.