Input/output analyses of digraphs representing dynamic systems are developed and presented. The relevant literature in control and system theory is reviewed. This review leads to the use of input and output reachability as being particularly appropriate for input/output analyses. Methods and algorithms are presented for determining input reachability and output reachability. These notions are applied to Forrester's WORLD2 model. Relationships between input/output reachability and the level partition are fully developed. Implications for input and output selection and analysis are given.