This article looks at the history of military manuals and their role in the dissemination of the law of armed conflict. It divides manuals into three categories, international, national, and internal, examining each in turn in relation to its role. The author concludes that there is a tension between the role of a manual as evidence of ‘state practice’ in the development of customary international law and the need to provide service personnel with clear understandable instructions for use in military operations. He counsels against excessive reliance on manuals as evidence of state practice as this could have the effect of making states reluctant to openly publish such documents with a detrimental effect in the operational field.