This book has two goals. It introduces a pattern of 4 interlocking constraints which I call the "structure of concern" and it issues a challenge to all of the thinkers of world to find the best level of description for it; the level at which it might be explained... concern structure models turn up everywhere, including discussions of knowledge management methodologies, suicide, yoga, information systems, sex, multi-agent networking, ethics, nervous system organization, drama, military planning, speech pragmatics, forest conservation, education and even philosophy. Some concern structure models are quite specialized and obscure, but some others count among the most widely used conceptual frameworks we have. My main goal in this book is simply to compare all of these frameworks to point out the similarities between them. This "catalog" itself is the argument I make in this book - the argument that some universal pattern lurks among all these models - a universal pattern that needs description.