Current methods for the nondestructive assay of special nuclear materials (SNM) and trans-uranic (TRU) waste in 208- drums can give assay errors of 100% or more when the drum matrix and/or radionuclide distribution is nonuniform. This problem is addressed by the development of the tomographic-gamma-scanner (TGS) method for assaying heterogeneous drummed SNM/TRU waste. The TGS method improves on the well-established segmented-gamma-scanner (SGS) method by performing low-resolution tomographic emission and transmission scans on the drum, yielding coarse three-dimensional images of the matrix density and radionuclide distributions. The images are used to make accurate, point-to-point attenuation corrections. The TGS geometric counting efficiency is 60% that of a typical SGS device, allowing a TGS assay time of only 28 min/drum with a one-detector system. The TGS method may also be useful for nondestructive examination. Currently, TGS is the only practical method of imaging SNM in drums.