The results, which should be a source of information, are obtained by measurement. To do this, apply the appropriate analysis tools, which include analytical procedures and the apparatus. These tools must be properly prepared to work so that the results are valid; that is, to reflect in an accurate manner (to be, at the same time, precise and correct) the actual content of the analyte in the test samples. In this chapter, the importance and the role of quality in analytical measurement is presented. The quality control and quality assurance system in analytical laboratory is described. All elements of QA/QA system and correlations between them are characterized.