 Questions about the nature of concrete computation should not be confused with questions about computational modeling. The dynamical evolution of many physical systems may be approximated by computational models—computer programs that simulate or predict the dynamics or kinematics of some system of interest. The behavior of rocks—as well as rivers, ecosystems, and planetary systems, among many others—may well be modeled computationally. It does not follow that the modeled systems are computing devices—that they themselves perform computations. Prima facie, only relatively few and quite special systems compute. Explaining what makes them special—or explaining away our feeling that they are special—is the job of an account of concrete computation.