Several members of the IAHR section on the use of computers in hydraulics and water resources have expressed concern at the quality of many of the computational models currently used in hydraulic research and hydraulic and coastal engineering practice. The purpose of this article is to explain some of the grounds for their concern by illustrating some of the errors that commonly occur in this type of work. Since the article is intended only as a popular review, no attempt is made to provide a detailed treatment of problems, (which, indeed, already provides material enough for several extensive treatises, such as those of RoACHE, 1972, and MAKCHUK, 1974).