This article describes the development of foreign fighters’ preparatory modes of operation between 2000 and 2013, based on an analysis of 17 closed police investigations and 21 semi-structured interviews with police investigators, public prosecutors, and lawyers. Through the use of grounded theory methods and a crime script analysis, we find that the phenomenon is not as new as is often portrayed. It changes over time as changing opportunity structures have an impact on the activities foreign fighters undertake during the preparation phase. We demonstrate how geopolitical changes, social opportunity structures, and technological developments affect themodus operandiover time. One of the implications of our findings is that the dynamic nature of the foreign fighting phenomenon requires flexible and tailored prevention measures.