1. Bodily Systems and Medical Ontology The Institute for Formal Ontology and Medical Information Science in Leipzig is constructing a reference ontology for the medical domain. It is designed not as a computer application in its own right but as a framework of axioms and definitions relating to such general concepts as: organism, tissue, disease, therapy. Here we focus on the concept bodily system, which we believe will serve as a central factor in a robust ontology of the human organism. The division of the body into its major bodily systems is by no means unproblematic. The National Library of Medicine 1 lists eight body systems, including the urogenital system; Wolf-Heidegger’s Atlas of Human Anatomy 2 lists only seven systems, etc. Standard sources often divide systems into three groups of supportive systems (the integumentary and musculoskeletal systems), exchange systems (the digestive, respiratory, circulatory, and urogenital systems), and regulatory systems (the nervous, endocrine, and immune systems). However there are elements of exchange systems (for example parts of the liver and pancreas) which play a role also in regulatory systems, and the three regulatory systems themselves effect their functions of regulation via a certain sort of substance-exchange.