The current concept of good manufacturing practices (GMP) emphasizes that the quality of pharmaceutical products should be constructed during the overall process cycle. Quality control department plays an important role in the quality-by- design (QbD) concept since it demands the acquisition of in-process reliable analytical data. Quality control department should demonstrate the quality of their results and their fitness for purpose by giving a measure of the confidence that can be placed on the results. One useful measure of this is measurement uncertainty. The present article covers the sources, reasons and evaluation of measurement uncertainty.