This monograph, which the author submits as a Thesis for LL.D. degree of the University of London, deals with a controversial subject. In fact, the problem of application of private law in international law is so controversial that in the course of the writing of this thesis doubts were frequently arising in the mind of the author whether the subject ought to be dealt with in a University dissertation. These doubts have rather increased than diminished since he decided not to confine himself as a mere registration of opinions of publicists and writers of text-books, but to examine whether the current opinion is in accordance with the practice of states, and whether it has been really incorporated into the science of international law.