Some time ago I was invited to give a lecture on “sludged blood” to the members of the Interstate Postgraduate Medical Association of North America and to prepare a paper on the same general theme to be published in Postgraduate Medicine. Reflection quickly showed that the verbal presentation of ideas about sludge to an audience is quite a different thing from a proper, written, general introduction to the subject of sludged blood. Consequently, the verbal presentation was confined to a short talk with a great deal of illustration, consisting largely of colored motion pictures.The films showed the specific characteristics of healthy normal unagglutinated blood and vessel walls as photographed in the omental vessels of healthy rhesus monkeys.** The illustrations of some of the kinds of damage done to the body by sludged blood were photographed in two pathologic conditions: (1) in rhesus monkeys with knowlesi malaria,† and (2) a more recently prepared motion picture‡ showing the sludge developed throughout the whole circulatory system of dogs following severe local thermal burn. We now believe this special type of postburn sludging to be a major causal factor in postburn circulatory shock, in postburn toxemia and in postburn anemia.This article is intended to introduce the reader to the literature describing observations, experiments and deductions on normal and pathologic circulatory physiology, particularly those dealing with sludge.