Full Description ADOPTED_FROM:CEN/TS 16800:2020 This document describes an approach for the validation of physico-chemical analytical methods for environmental solid matrices and water. The guidance in this document addresses the initial description of the method and two different validation approaches, in increasing order of complexity. These are: a) method development, if the method is developed by the laboratory, or conditions of adoption, if the method is a standardized protocol adopted by the laboratory; b) validation at the level of single laboratories (within-laboratory validation); c) method validation at the level of several laboratories (between-laboratory or inter-laboratory validation), with a focus on methods that are sufficiently mature and robust to be applied not only by a few expert laboratories but by laboratories operating at the routine level. The concept is strictly hierarchical, i.e. a method shall fulfil all criteria of within-laboratory validation before it can enter the validation protocol of the between-laboratory. This document is applicable to the validation of a broad range of quantitative physico-chemical test methods for the analysis of water (including drinking water, surface water, groundwater, waste water, marine water), and of solid environmental matrices, such as soil, sludge, liquid and solid waste, sediment and biota. It is intended for standardized protocols adopted by a laboratory, and either for test methods aiming at substances that have recently become of interest or for test methods applying recently developed technologies. The minimal requirements that are indispensable for the characterization of the fitness for the intended purpose of an analytical method are: selectivity, precision, trueness, performances characteristics and measurement uncertainty. The aim of validation is to prove that these requirements are met. In this document after the definitions (Clause 3) and description of the principles (Clause 4) a toolbox is given describing the relevant performance characteristics in the validation process. Clause 7 and 8 focus on the within laboratory validation process (V1) and Clause 9 on the interlaboratory validation process (V2). Clause 7 and 8 describe largely the same processes, but differ in approach for establishing the LOQ. Reporting of the results of the validation studies is addressed in Clause 10.