Induced-polarization (IP) effects in electrical geophysical surveys have been used in mineral-exploration projects for many decades, but only in the past 15 years or so has IP in environmental geophysics become a more common, commercially applicable method. This is primarily because of improvements in the economics of the technique and in data quality resulting from equipment advances. One field of environmental applications in which the IP method has been particularly useful is the study of buried waste. In the past, IP data acquisition was relatively expensive, and as a result, many landfill IP surveys were limited in scope for budgetary reasons. With the improving economics of the method, however, extensive amounts of data have been acquired at more than 50 landfills. At one site alone, 95 lines of IP and resistivity data were acquired, and at another, more than 14,000 transmitter-receiver pairs were used. Examination of this large, diverse data set allows several interesting general conclusions to be drawn that were not evident from the more limited surveys in the past. For example, early interpretations of IP results over landfills assumed that the IP effect was primarily the result of metallic debris in the waste. However, from the large database over many landfills, it appears that multiple sources of the IP effect are associated with the waste, and IP anomalies are clearly evident over landfills that contain little or no metal. It is also clear that IP surveys are substantially more reliable than other more traditional waste-mapping methods such as resistivity (or its inverse, conductivity) and magnetics. In addition, comparisons of data sets over time suggest that the IP method might be a useful tool in monitoring the degradation of subsurface waste.