This Issue Paper summarizes the main findings of the first workshop of the International Forum for Expert Exchange on Countering Islamist Extremism (InFoEx) workshop series. The workshop’s goal was to facilitate the exchange of lessons learned, inspiring practices, and knowledge gaps from programs and measures on countering Islamist extremism that are implemented in Denmark, France, Germany, the United Kingdom, and the Netherlands. This Issue Paper explores (common) challenges as discussed in the workshop. Furthermore, it provides three "flashlight" cases from Germany, Denmark, and the Netherlands. They serve as examples of specific approaches discussed during the workshop. The workshop was based on a needs-based approach and focused on the following five main topics: 1. Planning, financing, implementing, and evaluating tertiary prevention programs; 2. Multi-agency cooperation and clarity about roles between different actors and stakeholders; 3. Returning foreign fighters, focus on women and children; 4. Role of mental health and psychological factors in (de-)radicalization; 5. Effectively communicating tertiary prevention to the public and stakeholders.