This European Standard specifies general requirements and performance test procedures for portable and fixed position measuring devices (MDs) that are used in an in-line or online operating position to measure physical and chemical determinands in water. It excludes at-line devices, such as chemical test kits, and off-line devices, such as laboratory analysers. The general requirements include functional facilities that MDs need to meet users’ applications and information that need to be included in associated documents. The test procedures specify uniform methods to be used when determining key performance characteristics of MDs. The performance tests comprise testing carried out under laboratory and field conditions. Statistical procedures are defined for evaluation of the test data. It is recognized that for some devices certain test procedures are not applicable. Example values for performance characteristics for a selection of MDs for monitoring waste water effluents and receiving waters are detailed in Annex A for guidance. This European Standard requires the manufacturer of a MD to provide more technical data for verification than does EN ISO 15839:2006 [5]. Consequently, EN ISO 15839 will be of greater assistance to manufacturers wishing to characterize a new device whereas this European Standard is more focussed on user requirements for the verification of manufacturer’s claims.