… we do not know a truth without knowing its cause. Aristotle Perhaps the greatest hope that may be entertained for a scientific work, whether experimental or theoretical, is that it leads to new thoughts and new avenues of investigation on the part of its readers. In microvascular mechanics, the interplay of rheology, anatomy, and cellular and organ function has only just begun to be addressed. To understand the operational behavior of microcirculation, there is a need to integrate studies at the cellular or molecu lar level with a quantitative, biomechanical description of the circulatory system. The symposium entitled Frontiers in Cardiopulmonary Mechanics held in June 1988 at the University of Virginia was intended to provide a fundamental approach to the description of the circulation from the per spective of microvascular mechanics and to examine new methodology that may advance this effort. This book arose out ofthe work presented at the symposium. Aristotle expressed well the need to pursue the causes of a phenomenon in order to achieve a truthful understanding of its nature. In this spirit has each of the quantitative sciences progressed, and in this spirit we hope that this book will provide some understanding of the microvascular events and bio mechanical mechanisms underlying the behavior of circulation in general, and of pulmonary and skeletal muscle microcirculation in particular. The integrated treatment of pulmonary and systemic microcirculation provided here is intended to encourage the cross-fertilization of these two research fields.