The relative merits of different methods of sample preparation, of different instrumental operating procedures, and of different calibration techniques for major and minor elemental analyses of silicates must be judged by adequate estimates of precision and accuracy. Sources of imprecision in the X-ray method include electronic deviations, counting statistics, and sample collection and preparation. The variations attributable to these sources set limiting values to any X-ray method. Because most X-ray methods are comparative and thus dependent upon standards, the accuracy is more difficult to establish. Sources of inaccuracy in calibration methods include biases introduced by instrumental design or operation, matrix and particle size effects reflected as scatter or calibration line curvature, and an inadequate knowledge of the chemistry of the standard or standards used. With an adequate estimate of the precision, a practical measure of the accuracy of the X-ray fluorescence method can be obtained by the predictions of the compositions of standard silicates—standards whose compositions are assumed to be established by repeated wet-chemical analyses. This practical approach to an assessment of errors suggests to us that the comparative X-ray methods are highly precise. Within the limits of precision and with appropriate standards, accurate predictions of the compositions of silicate rocks can be made without applying empirically or theoretically derived correction factors. In this paper tests of precision and accuracy will be described which have been used to assess variabilities in quantitative analysis from the sample collection, through the sample preparation, to the calibration with standards.