This Research Paper explores how militant Islamists use propaganda to appeal to and radicalise supporters. It offers an alternative to the view that ideology is the key to understanding and countering the appeal of militant Islamism. Drawing on studies from the behavioural and social sciences, it analyses how strategies of meaning, credibility and behavioural change are deployed in militant Islamist propaganda citing a range of primary source materials – from the speeches and writings of charismatic figures like Abdullah Azzam, Osama bin Laden and Anwar al-Awlaki to messages released by the likes of al Qaeda, its affiliates and so-called “Islamic State”. This study particularly focuses on how such messaging leverages psychosocial forces and strategic factors to: (i.) provide its supporters with a system of meaning that shapes how they perceive the world, (ii.) demonstrate that it is a credible source of information and authority, and, (iii.) deploy pertinent behavioural levers designed to compel its audiences to legitimise and engage in violence. This Research Paper concludes by outlining some crucial lessons for understanding both the siren call of militant Islamist propaganda and counterterrorism strategic communications design.