IHL has often been deemed outdated and frozen inside a rigid framework, unable to provide an adequate response to the changing reality of armed conflict. The present article contests such a view, arguing that IHL is an expanding corpus juris with legal means to address these challenges. The authors identify two main currents of IHL expansion: implementing legislative measures, which comprise all measures taken at the national level to implement existing IHL rules; and developing legislative measures, which come into existence at the international or national level as a response to the specific challenges – including international treaties focusing on specific topics, customary rules, decisions of international tribunals, and national legislation and jurisprudence. The authors further examine the effect of these measures on IHL, arguing that national implementing measures and developing measures taken at the international level have resulted in the advancement and strengthening of IHL rules, as well as in an increase in the protection afforded by these rules. By contrast, the authors conclude that developing legislative measures adopted by some States at the national level are inconsistent with the main principles of IHL, and have an inhibiting effect on compliance with IHL rules.