Since Body Section Radiography, now generally known as Tomography, was introduced, a fairly large amount of literature ha grown around this special method of radiography. ?onographs have been written on this subject by EacDougall and Weinbren,who give a general account of its applications, but do not attempt an historical or technical survey. i,part from these two authors and Twining, the great majority of publications have been from foreign sources. This is hardly surprising, since all the initial develotments were carried out on the Continent and in America. In my radiological practice Tomography is employed to a considerable extent as a method of investigation. about two years ago I heard that a 'new' method for obtaining horizontal body sections by radiography was being carried out in Italy. I knew that an apparatus for this purpose had been constructed in England in 1939, and following trials in several hospitals had produced disappointing results. Suspecting that the latter may have been due to to the lack of facilities in overworked x -ray departments early in the war, I determined to carry out experiments in our department. The Medical Supply Association, Ltd., kindly placed their apparatus at my disposal, and having worked out the principles, this never having been properly done before, I was able to produce radiographs of horizontal sections of the thorax of good diagnostic quality and far surpassing the results previously obtained. It is the purpose of the first part of this Thesis to consider briefly the history of Tomography together with the principles involved and its role as a diagnostic procedure. The second part deals with the history of its development in the horizontal plane. The apparatus which I am using is described in detail and the technique of its employment is given. Finally, after describing the anatomical features shown in the normal thorax by this method, a number of cases of interesting intrathoracic lesions are demonstrated.