This Chapter concerns the formation (genesis) of a treaty. It deals with the substantive and formal elements of a treaty, ie when international obligations emerge through a treaty, when is a treaty a treaty, and who is entitled to bind a State through a treaty. This Chapter examines numerous cases by international courts and tribunals relating to this subject matter. It also analyses the debate ssurrounding memoranda of understanding (MOUs), soft law and modern law-making, such as the Paris Agreement on Climate Change, which comprises hard and soft law obligations. Finally, it looks at the registration of a treaty, ie to what extent, if any at all, it contributes to its formation and identification.