DISEASES involving alterations in the hemostatic mechanism contribute an important percentage of morbidity and mortality as seen today. Such disorders may be due to inadequate hemostasis and be manifest by excessive bleeding, or the reverse may be seen—intravascular clotting, thrombosis, and embolism. The aims of this presentation are: to define the possible causes of bleeding, to discuss the normal hemostatic mechanisms, to classify the hemorrhagic diseases, to outline some of the clues which aid in differential diagnosis, and to describe some of the research approaches to the unsolved problems in this field. The possible causes of excessive bleeding are: Vascular Abnormality.— The most obvious possibility is some abnormality in the vascular system which would allow blood to leak out, perhaps analogous to the rusty pipe which springs a leak at the slightest injury. Tissue Abnormality.— Although we seldom think of tissues as important to hemostasis, they do serve