A much more complete theory of transient flow in leaky aquifers has recently been developed and can be applied to the problem of investigating the hydraulic properties of caprock layers over potential gas storage reservoirs. During a pumping test, the transient behavior of a multiple-aquifer system depends on the properties of the aquifer being pumped as well as the degree of communication that can develop across the caprock to unpumped aquifers. However, at sufficiently email values of time defined by tD 1. 6 b2/(r/B)\ the effects of the unpumped aquifer can be ignored and the caprock can be treated as though it were semi- infinite. The application of this new theory to the problem of testing aquifer gas storage prospects is reviewed, and it is shown that the theory provides a better understanding of earlier concepts on slightly leaky caprocks. The general applicability of the method is demonstrated by analyzing data from field tests on two different aquifers with very leaky caprocks. The results are in excellent agreement with core analyses of the caprocks.