Under the United Nations Convention on the Law of the Sea (UNCLOS), four tribunals are provided to interpret and apply the Convention. When it was concluded, the enumeration of the four tribunals, including permanent courts such as the International Court of Justice (ICJ) and the International Tribunal for the Law of the Sea (ITLOS), was said to lead to the risk of fragmentation of UNCLOS. However, so far, they seem to have successfully avoided such risk by interpreting and applying the Convention harmoniously, in other words, through the harmonised operation of UNCLOS. Meanwhile, the fact that the operation of UNCLOS is still at an early stage should be noted. The more cases these tribunals handle, the more flexibility they lose. This loss of flexibility may become a new cause of fragmentation of UNCLOS, because if each tribunal is bound by its own case law and adheres to it, it becomes difficult for each tribunal to take into account the decisions of other tribunals and operate UNCLOS uniformly. Against this background, this chapter contrasts the practice of courts and tribunals established under the UNCLOS in relation to cross-fertilisation and consistency in legal findings. This chapter also discusses how harmonised operation of UNCLOS can be reconciled with differences in the practice of the UNCLOS tribunals.