This work had its inception in 1945 when C. and O. Vogt suggested to one of us (J.O.) the need for investigation of the reticular formation of the brain stem. The Vogts felt that their studies of the extrapyramidal diseases, limited to an investigation of the basal ganglia, had illuminated only one aspect of the problem. In their opinion thorough knowledge of the morphological and functional organization of the reticular formation was indispensable for the understanding of the regulation and co-ordination of motor activity by the extraphyramidal systems. Accordingly, a study of the normal cytoarchitecture of the reticular formation was commenced at the Brain Research Institute, Neustadt, with the intention of applying this knowledge in the investigation of pathological material at a later date. For various reasons this work was interrupted for a period of several years, and it was not until 1952 that it was resumed at the Montreal Neurological Institute. It soon became apparent that it was advisable to extend the scope of the project to include the cytoarchitecture of all the gray masses of the lower brain stem. This conclusion was based largely on the lack of precise definition of the boundaries of the reticular formation and the insufficient data available concerning the cytoarchitecture of many nonreticular nuclei. At the present time the most comprehensive descriptions of the cytoarchitecture of the human brain stem are to be found in Jacobsohn's Uber die Kerne des menschlichen Hirnstamms " (1909) and in Ziehen's "Anatomie des Centralnervensystems" (1933). Jacobsohn's widely utilized drawings of cross sections of the brain stem constitute the most accurate guide at present available for delineation of the various nuclear masses. These drawings are necessarily overschematized due to their small size, and neither they nor the descriptions of nclei are supplemented by photomicrographs. Ziehen's exhaustive description of the cyto- and myeloarchitecture and fiber connections of the brain stem, supplemented by an extensive bibliography, suffers from the lack of representative serial cross sections stained for nerve cells. In addition the photomicrographs presented are limited to a few myelin stained preparations. Marburg devotes a considerable part of his atlas to the description of the cytoarchitecture of the human brain stem, but presents only five schematicrepresentationn of Nissl stained cross sections and includes no photomicrographs of this region. More cyoarchitectural detail of various regions of the human brain stem may be found in the monographs of Gagel and Bodechtal (1930), Stern (1936) and Crosby and Woodburne (1943), all of which are illustrated by excellent photomicrographs. Riley's "Atlas of the Basal Ganglia, Brain Stem and Spinal Cord" (1943), Although not directly concerned with cytoarchitecture, should be mentioned as an invaluable guide to any student of these regions. Apart from these comprehensive reviews, innumerable other investigators have confined their studies to the anatomy of individual brain stem nuclei. The presents work is an attempt to portray adequately and objectively the cytoarchitecture of all the nuclear structures of the medulla oblongata, pons and midbrain. In addition to the presentation of formerly recognized nuclei, several previously undescribed cell groups have been delineated on the basis of cytoarchitectonic criteria. The majority of these lie within the reticular formation. The contents of the monograph fall naturally into two parts. The first consists of a series of nineteen semischematic representative cross sections of the brain, stem, accompanied by low power photomicrographs. Descriptions of the individual nuclei, supplemented by photomicrographs of higher magnifications, compose the second part. It is hoped that this atlas will prove of value to the neuroanatomist who is interested in the position and morphology of the individual nuclei, and to the neuropathologist in his attempts to localize pathological processes and to distinguish abnormal cell forms from the confusing array of morphologically different normal cells found within various regions of the brain stem. Further, the neurophysiologist may find it useful to have available detailed human morphological data which can be correlated with that of experimental animals, and with which his functional concepts may be integrated.