Preventing violent extremism is typically seen as a task for government security agencies, and yet civil society organizations also play a vital role. From community networks operating at the grassroots to policy research institutes, non-governmental groups have unparalleled advantages in reaching marginalized groups and supporting effective, locally defined responses. What are the most effective ways of supporting civil society efforts to prevent violent extremism across Southeast Asia? In seeking to answer this question, The Asia Foundation interviewed specialists, practitioners, and community members across four countries—Indonesia, Malaysia, the Philippines and Thailand—and identified common lessons. Many of the lessons stress technical points of interest to foreign aid agencies and national governments, such as the value of identifying joined-up and intensive approaches for high-risk sectors and the need to adopt a strategic portfolio approach which balances existing counterterrorism, peacebuilding and development interventions. There is also a need to respond to local context. Violent extremism may be associated with international networks, but also with domestic movements, internal conflicts, or growing intolerance between ethnic or religious groups. The implications and sensitivities of these associations in each locale are critical for policy and programming, and a failure to appreciate them can affect the success of programs or even do harm. Meanwhile, growing restrictions on civil society organizations in many countries limit their scope to engage effectively.