To produce reliable analytical data in a laboratory, all steps of the analytical chain have to be properly understood and controlled. Sampling, sample pretreatment, storage, subsampling, dissolution and/or mineralization, choice of analytical method, calibration of the measuring instruments, and metrological evaluation of data should all be subject to QC (quality control) and QAS (quality assessment) measures. QC includes measures for minimizing errors and keeping them within the required limits. QAS measures are monitoring the quality of the produced data. Some aspects of QC and QAS are described and examples are given of the practical difficulties which may occur in an analytical laboratory.