According to ISO/IEC 17025 (3): The laboratory shall have quality control procedures formonitoring the validity of tests undertaken. The resulting data shall be recorded in such a way thattrends are detectable and, where practicable, statistical techniques shall be applied to the reviewingof the results. The monitoring shall include e.g. regular use of internal quality control. … Qualitycontrol data shall be analysed and, where they are found to be outside pre-defined criteria, plannedaction shall be taken to correct the problem and to prevent incorrect results from being reported.Internal quality control at the chemical analytical laboratory, involves a continuous, criticalevaluation of the laboratory’s own analytical methods and working routines. The controlencompasses the analytical process starting with the sample entering the laboratory and ending withthe analytical report. The most important tool in this quality control is the use of control charts. Thebasis is that the laboratory runs control samples together with the routine samples.The results of the control program may be used in several ways - the analyst will have an importantquality tool in his/her daily work, the customer can get an impression of the laboratory’s quality andthe laboratory can use the results in the estimation of the measurement uncertainty.The QC has to be part of a quality system and should be formally reviewed on a regular basis. Theaim of this handbook is to describe a fit for purpose system for internal quality control at analyticallaboratories that are performing chemical analysis. The approach is general, but the examples aremainly from environmental analyses.