mental standards, and by statistical and systematic errors in the measurement of x-ray lines and in the separation of line intensity from spectral background. The errors in estimating the characteristics of the detector system, with exception of deadtime effects, cancel when the emission from the specimen is divided by that from the standard. The estimate of achievable accuracy of EPMA and the comparison of diverse approaches must therefore involve consideration of theoretical aspects, of errors in the values of parameters used in the procedure, and of operational errors in specimen and standard preparation and in analysis. In the development of current "correction procedures," theoretical, mathematical and measurements aspects are interwoven. For instance, the distribution in depth of x-ray generation can either be calculated by Monte Carlo calculations, which are limited by the accuracy of available parameters, or by exeriments with sandwich tracer targets. To further complicate things, some Monte Carlo calculations use techniques empirically adjusted to fit the available experimental evidence. Since the precision of measurement of relative x-ray intensities is better than the accuracy of results on specimens of known composition, there is room for improvement; but, to achieve this, the diverse potential source of errors will have to be unraveled and tested separately.