A FEW months after the discovery of X rays the first communications of X-ray findings in diseases of children were published ; and shortly after the conception of ' clinical rcentgenology ' had been defined, a monograph on all that had been achieved so far was soon available (Reyher, Die roentgenologische Diagnostik in der Kinderheilkunde Ergebn. inn. Med. Kinderheilh., 19o8 , 2). The many text-books of pmdiatric roentgenolog~Rot ch, i9IO ; Reyher, 1912 ; Gralta, i927 ; Becker, i931 ; Engel and Schall, 1933 ; and especially the standard text-book by Caffey (i945) point to the rapidly increasing extent of this chapter of clinical roentgenology. Without doubt radiology of infancy and childhood requires special study, and is even developing into a specialty of its own. Caffey (I948), giving an account of a number of the achievements of pa~diatric roentgenology during the past few years, expressed similar thoughts in an editorial. Although all these papers and text-books deal with paediatric roentgenology as far as it deviates from general roentgenology, there has never been given--as far as I can ascertain--an explanation of the nature of this difference. Surely there are no technical peculiarities justifying a separation of p~ediatric roentgenology from general roentgenology, as the technique may be mastered without too much difficulty. Nor can p~ediatric roentgenology be considered to be a specialty like radiology of the chest, the gastrointestinal tract, the renal tract, or the nervous system, which are anatomical subdivisions. What makes pa~diatric radiology a specialty are the biological peculiarities of the newborn, the infant, and the child, the very same factors which make p~ediatrics a specialty of general medicine. As a consequence of these biological peculiarities there are diseases occurring in infancy andchildhood not to be met with in adults, and there are also diseases, though common to all ages, which take a different course in the child. Attempting to analyse these biological peculiarities, I feel entitled to do so as a radiologist, not only because their knowledge is essential for an understanding of the special position of p~liatric radiology, but rather because some of the biological elements of children's diseases owe their discovery to radiological research. Quite a number of diseases of childhood have either been discovered or recognized as a specific disease entity by means of radiology. Here I naturally only wish to deal with those factors which affect the X-ray pathology of the child, i.e., factors which have made X-ray paediatrics a specialty. These factors are :I. The embryological development, which may be disturbed by many factors. Mutation may be caused by injuries to the genes, or by intra-uterine injuries ; in cases of deficiency of liquor amnii or amnion cordis. They all cause maldevelopment, the recognition and exact definition of which represent a large chapter of pmdiatric radiology. Some of them are to be recognized during life only by means of radiology, e.g., hemivertebra as a cause of scoliosis, spina bifida, and other deformities. 2. Hereditary factors play a decisive part in quite a large number of malformations, e.g., in chondrodystrophy, fragility of bone, cartilaginous exostosis, Mandelung's deformity, marblebone disease, congenital cystic disease of the pancreas, and many others. The recognition of the familial incidence as well as the cause of the hereditary transmission are largely due to X-ray examination, especially by revealing abortive cases in parents and other family members.