The author has followed the outline of his book "The Pneumonias," published in 1938, with newer facts and concepts included to bring the subject up-to-date. In general, the pneumonias are divided into those (1) caused by specific infectious agents, (2) that occur as part of systemic infectious diseases, (3) of mixed bacterial origin secondary to other conditions, and (4) caused by allergic, chemical, and other factors. This book should appeal to any physician treating pneumonia. It is practical in its approach to the general problem, and the most space is devoted to the usual types of the disease. Viral pneumonia is discussed in the light of our present knowledge, and this includes a group of pneumonias of probable viral origin. Each type is usually presented from the standpoint of etiology, pathogenesis, pathology, symptoms, prevention, and treatment. Antibiotics and their effect on the disease are always considered.