McNair's view may lead the reader to turn to the next chapter; and it has to be said that, although the relationship between successive treaties can be immensely difficult, it does not cause daily consternation in the legal departments of foreign ministries. But, since McNair wrote in 1961, the topic has gained in importance. Because of the greater number and complexity of multilateral treaties, particularly on the environment, questions increasingly arise as to the relationship between successive treaties. In recent years, more has been written on the problem. But it is the sort of puzzle that is difficult to study in the abstract; it can be done properly only in relation to actual treaties and real problems. This chapter will therefore concentrate more on what rules there are, and practical ways of seeking to overcome future problems.

There should be no problem when the parties to both treaties – whether bilateral or multilateral – are identical. If the earlier treaty is not considered as terminated or suspended (Article 59), the earlier treaty applies only to the extent that its terms are compatible with those of the later treaty (Article 30(3)).

Problems can occur when two or more multilateral treaties deal in whole or in part with the same subject matter, but the parties to them are not identical.