25.1 A critical feature of many legal systems is the prohibition of relitigation of a cause of action or issue upon which judgment has been pronounced in a case in which the person seeking relitigation was a party. In English law the principle is implemented generally through res judicata but appears on occasion as cause of action or issue estoppel or simply abuse of process.1 Essentially connected with it is the concept of merger of a cause of action into a judgment so that the judgment becomes the sole source of the remedy. There may well be a critical distinction between merger and estoppel in that merger destroys the cause of action while estoppel can be countered by a contrary estoppel. So the party relying on issue or cause of action estoppel may itself be estopped by conduct from that reliance—as, for example, where the parties have agreed that the action should have limited effect on any further action.2