Many authors have deplored the scarcity of evaluations that assess the impact and effectiveness of programmes and policies in the field of Counter-Terrorism (CT) and Preventing/Countering Violent Extremism (P/CVE). This makes it difficult to understand which efforts to address violent extremism have had positive results, and what measures and methods have been effective in identifying impact. This policy brief aims to provide policymakers and practitioners in the CT and P/CVE field with the necessary background to integrate monitoring and evaluation (M&E) in their work. It explains what M&E is, why it is important, and how to start or advance M&E in your own institution. The key recommendations are: 1) to allocate five to ten percent of the budget to M&E, 2) to integrate M&E from the programme or policy design phase onwards, and 3) to create an enabling environment for M&E.