Digital computer simulation methods were used to examine the effects of storage in urban storm water runoff systems. Two specific cases of drainage problems are examined. The first is the problem of designing a storm drainage system that flows into a reservoir from which the runoff is pumped or flows by gravity to a receiving water body with a variable stage. The second case is that of determining the storage required for peak storm water runoff volumes to achieve a fixed outflow from a watershed. Examples illustrating each case and suggesting the best design approach for similar storm water drainage facilities are given. For the fixed outflow case, a relative cost comparison is undertaken to determine the possible economic advantages of including storage in urban storm drainage systems. A computer model was developed to assist a designer in evaluating the amount and return of transient storage within a proposed design system.