In their quest to find ways to reduce civilian casualties during armed conflict, States often emphasise the importance of compliance with fundamental rules of international humanitarian law that apply during the conduct of hostilities. Chief among them are the rule of distinction, proportionality and precaution. This contribution focuses on the proportionality principle. It examines whether there is a need for clarification or development of this rule. After highlighting reasons why clarification of the law on proportionality is necessary, the author proposes a guidance document on proportionality decision-making in armed conflict. To lay the foundation for such a document, the author identifies a range of issues that could be addressed in the document.