The Vienna Convention on Succession of States in respect of Treaties had a very difficult task. It was aimed to find a balance between State’s will as the source of international obligations and the need for legal stability. Thirty years after the Vienna Convention, State’s compliance to its rules is not satisfactory. Almost worldwide, the subject remains ruled only by customary rules. Hence, to confirm the customary nature of the Convention we must examine State practice. This is the objective of the present article. To prove our argument, we begin a detailed analysis of the Convention, dividing its contents in two overlapping circles: the subject of the  treaty and the type of succession concerned. The whole of the Convention is formed by rules and exceptions taken from combinations of these two circles. This article unveils that structure and then compares the Convention’s rules with State practice, following the same method and the same typologies. Our conclusion is mainly negative, because State practice does not follow Convention’s rules in cases of incorporation, separation and secession. The situation is different regarding dissolution, fusion and decolonization.