Pressure build-up data in low permeability reservoirs take too long and are usually of poor quality. It is often not economically feasible to shut in a well with a high production rate for a buildup test. Sand producing wells are not good candidates for long pressure drawdown tests. It is often impracticable to maintain a constant rate long enough to complete a drawdown test. In these cases, a multi-rate flow test should be run instead of buildup or drawdown tests. Actually a well-designed multi-rate flow test may minimize the influence of wellbore storage on pressure data. A new technique based on the pressure derivative concept is presented for interpreting a multi-rate flow test. It is shown here that a Cartesian plot of the pressure derivative data versus a time group is a straight line from which the reservoir permeability can be estimated. It is also shown that for the case of two-rate test, Tiab's Direct Synthesis technique is applicable for calculating permeability and skin A step by step procedure is presented for interpreting a multirate test using pressure and pressure derivative data. This new technique is illustrated by several numerical examples.