Hydrologists unfamiliar with computer programing methods can use Hymo to great advantage in watershed modeling. This problem‐oriented computer language provides twelve commands, commonly used in hydrology, to transform rainfall into runoff hydrographs and to route these hydrographs through streams and valleys or reservoirs. These functions make Hymo quite useful in the design and evaluation of flood control structures, flood forecasting, and research studies. The procedures used in Hymo were selected because of their accuracy, simplicity, and practicality. The input data required for Hymo are normally available for most watersheds. Hymo is quite flexible in that hydrologists familiar with Fortran 4 programing can easily add new commands or modify the present command