A century ago, chemical weapons were used in World War I, with their use during the second battle of Ypres, in particular on 22 April 1915, demonstrating their nature as weapons of mass destruction. On 21 August 2013, during the Syrian civil war, sarin-filled rockets hit the Ghouta suburbs of Damascus, killing a large number of civilians. It is against the background of the indeed limited use of chemical weapons over the last century that the steps towards a nearly universal prohibition thereof are analysed. The starting point is early steps towards only prohibiting the use of chemical weapons (primarily focusing upon the 1899/1907 Hague Regulations and the 1925 Geneva Protocol) and their emerging customary law nature. With the adoption of the 1993 Chemical Weapons Convention, the prohibition of use was strengthened not only in scope but also by linking it to pertinent disarmament and arms control provisions. The latest steps address individual criminal accountability for using chemical weapons as a means of warfare, based upon the 1998 Rome Statute and the 2010 Kampala amendments thereto. As the concluding section illustrates, the effectiveness of a century of pertinent international law making depends on the universality of the prohibition to use chemical weapons and the common efforts of all stakeholders to ensure the integrity of the regime established by these various layers of international law.