Alternative Tools (AT) such as on-site test kits, on-site portable devices and continuous measuring devices, are useful to improve water quality assessment under EU directives, and water treatment processes, thus contributing to improving water management, as well as get insights on the dynamic of pollutants within water bodies. Although these tools have clear advantages (e.g. fast response allowing for real-time monitoring, ease-of-use, lower cost), they are perceived as less reliable than conventional analytical methods. Their alternative nature, innovative status, and non-standard operation mode require specific validation strategies that differ significantly from those of conventional analytical methods. Nevertheless, the validation of ATs, especially on-site test kits, portable devices and continuous measuring devices for water quality monitoring is crucial to support their acceptance, promote their use and make their application sustainable. In this paper, a validation procedure in 4 steps is proposed: (1) Comprehensive description of the AT; (2) Assessment of intrinsic metrological performance and operational factors within a single laboratory; (3) Assessment of inter-laboratory performances through an inter-laboratory comparison and, (4) Demonstration of the equivalence of results between the AT and a reference method. This paper discusses each step of the validation procedure, and examples to illustrate critical issues are provided.