International law-making has long been considered the exclusive preserve of States. However, it is increasingly recognized that this traditional view does not fully account for the complexities of international reality. International humanitarian law directly addresses non-state parties to armed conflicts, reflecting that they possess limited international legal personality. This thesis explores whether, and under what circumstances, the consequences of this limited personality extend to the capacity of such armed non-state parties to undertake legally binding acts, by analysing the unilateral declarations, agreements and internal rules and regulations of such groups. The analysis is undertaken using the civil law concept of the juridical act. The research demonstrates the usefulness of this general theory for assessing the legal value of undertakings by non-state actors. As such, this thesis is part of a growing body of international legal scholarship that investigates the boundaries with respect to the creation of international legal obligations.