This report establishes and details the clinical concept of the subperitoneal space and the subpleural space as components of the continuum formed by the subserous space. The subserous space is an anatomic plane formed in the embryo that persists during development into the adult. This is the basis of the thoracoabdominal continuum. The embryologic development is presented from the viewpoint not of the contents and their relationships within the developing coelomic cavity but rather that of a continuous space deep to the lining serous membrane. Clinical material is presented that illustrates different disease processes that can directly spread in either direction within this continuum. This unifying concept provides an understanding of the pathogenesis of direct spread of disease processes within and between the thorax and abdomen.