The subject of customary international law as a general phenomenon is hardly more suitable for graduate research students in international law than Fermat's last theorem used to be for their counterparts in mathematics. The central puzzles of a discipline, which generations of its senior professionals have failed to solve, are usually better approached from the edges, and indirectly. Light may thus be shed on the centre, but there is less risk of complete failure. So when Michael Byers came seeking to work on custom it seemed sensible to look not frontally at the ‘problem’ as such, but at a number of examples of different kinds of custom in transition, at different contexts where, we could be relatively sure from the communis opinio, a particular customary rule existed and had changed. What were the factors that had produced the change; how had they interrelated; what influence did the ‘structure’ of the particular problem exercise – for example, what difference did it make on the evolution of a particular institution or custom that the issue characteristically arose in one forum (national courts in the case of state immunity, foreign ministries in the case of the breadth of the territorial sea)? At least it was a starting point.

It says much for the energy and initiative of its author that the resulting book tackles these particulars within the framework of a study seeking to show the ways of international lawyers to the scholars of international relations.