This chapter explores the effects of the trust problem on jihadi internet discussion forums. The scarcity of non-verbal cues in digital communication facilitates deceptive mimicry, which undermines the inter- personal trust required for sensitive transactions. Open-source data from Arabic-language jihadi forums between 2006 and 2011 indicate that distrust there was high and direct recruitment rare. General trust also declined over time as policing of the forums increased. As of 2014, forums are still in use, but primarily for low-stake activities such as propaganda-sharing and ideological debate, not recruiting or operational coordination. Confidence in the authenticity of propaganda remained relatively high, due to vetting institutions and hard-to-fake video formats. A modicum of interpersonal trust also remained, thanks to reputation systems and a few relatively reliable signs of trustworthiness involving time expenditure. The trust problem is an Achilles heel for high-risk activists online, including pro-democracy activists in authoritarian settings.