Through the use of theoretical and field examples, the potentially large errors associated with use of straight-line p/z decline (tank) analysis to assess gas in place (GIP) and reserves in tight-gas reservoirs are illustrated, The basic tank assumption that reservoir pressure gradients are small can be violated, even with long shut-in times, It is also demonstrated that a reasonably-straight-line p/z decline does not necessarily indicate that the reservoir behaves as a tank. The communicating reservoir (CR) model is presented as a simple, yet much more accurate, method of performing material-balance calculations in tight reservoirs. The results of the application of the CR model to the Waterton gas field are used to illustrate the success and large impact that can be obtained by examining the pressure behavior more closely than simply plotting p/z.