The semi-input-output method was introduced by Professor Jan Tinbergen in the early 1960's in fairly obscure places in the economic literature. The basic idea of the method is a very simple one and does not require lengthy and sophisticated exposition to be understood. This fact, together perhaps with Tinbergen's dislike for very formalized and technical analysis, probably explains why he himself has never given a full exposition and elaboration of this method. Nevertheless, the concept did not remain unnoticed and authors such as B. Hansen and some of Tinbergen's collaborators, in particular P. A. Cor­ nelisse and J. Versluis, have contributed to the elaboration of the method. The present work by Dr. A. Kuyvenhoven, also a long-time collaborator of Tinbergen, has provided us with the first full, and in my opinion definite, exposition of the nature, technique and use of the semi-input­ output method, now more than 15 years after the launching of the concept.