It is not difficult to explain why it was decided to reproduce some of Einthoven's papers with photographic fidelity. The immense amount of information provided by electrocardiography, which in spite of its natural limitations has become one of the main diagnostic procedures in cardio­ logy and at the same time a cornerstone of scientific research, is universally acknowledged. On the other hand, we have also come to realize how important it is not only to know the historical perspective of cardiology, or any other field of medicine, but also to study the original sources from which our knowledge stems, without the contamination of later opinions or traditional interpretation, however well founded they may seem. * There is also a reason for the timing of this publication of facsimile reprints: Einthoven died 50 years ago; about 75 years ago the string galvanometer was invented and used for electrocardiography (first publication on the instrument in 1901, first publication of electrocardio­ grams taken with it in 1903). The papers, which are reproduced here, constitute a selection, necessarily limited and arbitrarily chosen. I should like to add, that the selection was made from several points of view. In the first place, only those articles were selected which had a definite and direct relation with cardiology, in particular electrocardiography. Although it is obvious from Einthoven's bibliography (chapter 2) that several of his papers deal with other subjects, the prominent place which electrocardiography occupies in Einthoven's work, justifies this choice.