This report explores how and why mis/disinformation develops in the wake of terror attacks and the ways it is used by extremist groups to attempt to shape public understanding and political responses. These uses include extremist sympathisers engaging in information manipulation and obfuscation as part of their attempts to explain or justify the violence, as well as distorting and deceptive messaging designed to marginalize or stigmatize other social groups. Having presented evidence and insight about the construction of these messages, the discussion also looks at the policy and practice options in terms of ‘what works’ with regard to managing and mitigating any such messaging and the harms it seeks to induce.