This paper considers the risk to major hazard plant from terrorists deliberately causing catastrophic industrial accidents. The United States of America Department of Justice [Assessment of the increased risk of terrorist or other criminal activity associated with posting off-site consequence analysis information on the internet, 2000] reports that “breaching a containment vessel of an industrial facility with an explosive or otherwise causing a chemical release may appear relatively simple to…a terrorist”. They concluded that the risk of such action is “real and credible”. Analysis of terrorism is often hampered by its being described as ‘irrational’; one corollary would be that it is unpredictable. However, terrorism may usefully be treated as a rational behaviour and in doing so it becomes possible to assess the risks it causes. We analyse the vulnerability of major hazard plant to terrorist attack and identify nine factors (access, security, visibility, opacity, secondary hazard, robustness, law enforcement response, victim profile, and political value) that might be used as a starting point for more formal risk assessment and manage.