The publications of the Research Group for European mi- gration problems represent studies by independent writers. These studies do not form an integrated whole, but are inter- related by their subject, namely, European and international migration. The topic is, therefore, approached in various ways, like a discussion in which experts from different spheres of activity expose their views on population in Europe. The writers not necessarily always agree in their opinions. The series must do surely be all the more valuable be ause of this, since the solution of the problem of over-population in Europe is of such great importance as to deserve a wide, multilateral approach. The author of the present publication, Mr. H. A. Citroen, is an official of the International Refugee Organization (I. R. O. ). It is not surprising that his approach should differ from that of Dr Hilde Wander, the author of the first publication of the series 1). Dr Wander has stressed the present demographic trends in the more important Western European countries, and the possibility of the absorption of an ever increasing number of workers into the economy of Europe itself. She is mainly thinking in terms of the integration of Europe. Mr. Citroen's idea, on the other hand, is that of "one world.