X-ray counting statistics plays a key role in establishing confidence limits in composition determination by X-ray microanalysis. The process starts with measurements of intensity on one or more samples and standards as well as related background determinations. Since each individual measurement is subject to variability associated with counting statistics, it is necessary to combine all of the counting variability according to established mathematical procedures. The next step is to apply propagation of error calculations to equations for quantitative analysis and determine confidence limits in reported composition. Similar concepts can also be applied to trace element determination. This approach can then be combined with spectral simulation modeling, making it possible to predict detectability limits without additional measurements.