This Technical Specification provides comprehensive guidance on within-laboratory and between-laboratory quality control for ensuring the production of results with a known level of accuracy in the analysis of waters. This Technical Specification is applicable to the chemical and physicochemical analysis of all types of waters. It is not intended for application to the analysis of sludges and sediments (although many of its general principles are applicable to such analysis) and it does not address the biological or microbiological examination of water. Whilst sampling is an important aspect, this is only briefly considered. Analytical quality control, as described in this Technical Specification, is intended for application to water analysis carried out within a quality-assurance programme. This Technical Specification does not address the detailed requirements of quality assurance for water analysis, which can be found in the EURACHEM/CITAC Guide (2002) [20]. The recommendations of this Technical Specification are in agreement with the requirements of established quality-assurance documentation (e.g. ISO/IEC 17025). This Technical Specification is applicable to the use of all analytical methods within its field of application, although its detailed recommendations may require interpretation and adaptation to deal with certain types of determinands (for example, non-specific determinands, such as suspended solids or biochemical oxygen demand, BOD). In the event of any disparity between the recommendations of this Technical Specification and the requirements of a standard method of analysis, the requirements of the method should prevail.