Abstract Experience with a number of material balance data sets from the field and from simulation has revealed some procedures that can be used to improve analysis of both oil and gas reservoirs: Failure to account for a weak water drive can result in material balance errors that are not insignificant.The assertion of previous authors that a weak water drive exhibits a negative slope on the Cole Plot (gas) and Campbell Plot (oil) has been confirmed. A weak water drive is detected on these plots much more unambiguously than on other common plots such as the p/z plot for gas.A Modified version of the Cole Plot is proposed to account for formation compressibility.The reservoir drive indices are a useful tool for determining the correctness of the material balance solution because they must sum to 1.0. The drive indices should never be normalized to sum to 1.0 because this obscures their usefulness and leads to a false sense of security.A modified version of the Roach Plot (for gas) is proposed that improves interpretation in some water drive situations.Material balance has not been replaced by reservoir simulation, rather it is complementary to simulation and can provide valuable insights to reservoir performance that cannot be obtained by simulation.