The recognition of normal and abnormal patterns in the breast by mammography is a skill that must be acquired through training and experience in the same way the radiologist learns to recognize the normal or abnormal skull, gastrointestinal tract, chest, spine, and other anatomy. To disseminate information on the breasts to radiologists on a large scale, the most effective method - person-to-person teaching - is obviously impractical. Even postgraduate courses in mammography can reach but a few fellow radiologists. Therefore we have felt for some time that a teaching Atlas of 1lalJlJJJography would serve a useful purpose, within the limita tions of the method, by bringing to the attention of radiologists and physicians in general the salient features of the X-ray appearance of the normal and the abnormal breast. In mammography as in no other area of roentgenologic study, technique is of crucial importance. Accurate diagnosis is abso lutely dependent upon demonstration of very fine detail, some times as delicate as a linear strand or two, or minute specks of calcium. To some extent the pictorial representation embraced by an atlas defeats the teaching objective, for much fine detail is often lost in reproducing radiographs. Every effort has been made to obtain the best possible reproductions of mammo grams in this Atlas) but some illustrations must of necessity remain unconvincing to the reader. Some of the details describ ed will be appreciated only when viewing a good quality original radiograph.