This article introduces the Terrorist Attack Complexity Index, a framework to measure the complexity of terrorist attacks. This measurement takes place on the basis of 16 characteristics of terrorist attacks. The framework is applied to jihadist terrorist attacks in Europe during the period 2004–2011 to find perpetrator characteristics that are related to attacks with a high degree of complexity. The findings suggest that complex attacks are more likely to be carried out by perpetrators, who have gained expertise by visiting training camps outside Europe or participated in actual jihad, worked in large groups, and are acting on instructions from other terrorist groups. Simple attacks are generally carried out by perpetrators who have none of these characteristics.