An overview of BIOMOD, an operational system designed to enable investigators to study models of dynamic systems, this paper presents a scenario describing the construction and simulation of a model of long delays at airports. BIOMOD employs a graphics console comprising a television screen, data tablet, and keyboard. A user constructs a model by drawing block diagrams and handprinting or typing text while receiving immediate feedback about the interpretation of his actions. Each component of a model's block diagram is defined either by another block diagram, or by one of the other user-oriented languages: analog-computer-like elements; algebraic, differential, or chemical equations; FORTRAN statements; or arbitrary graphical functions. During model simulation, displayed curves are automatically updated; the user may stop the simulation and plot different variables, change scales, or modify parameter values, and then continue. (Prepared for presentation at the Popov Society Congress, Moscow, June 1973.) 19 pp. Ref.