The historical development of knowledge of aneurysms is surveyed, particularly of those of thoracic aorta. There follows detailed considerations of the structure and structural properties of the aorta and its supports ("bobbins"). The degenerations of the aorta is described and discussed in considering the various basic types of aneurysm. Improved pathological techniques make possible greater differentiation of inflammatory, degenerative and other changes of the aorta media. These investigations allow the conclusion that the vast majority of thoracic aortic aneurysms are the result of "extras" consumed in and essentials left out of the modern way of life. Progress in preventive measures depends on better knowledge of the metabolism of the "bobbins".