The main conceptual approaches of water quality assessment and existing methods of standardization and classification of water on various parameters were summarized. Comparative analysis of sanitary and environmental principles of standardization has been fulfilled; the advantages and disadvantages of these approaches were discussed. It was concluded that the most reliable information about the state of the aquatic ecosystem can be obtained by using a physical, chemical and biological parameter complex. The international experience of water quality assessment was analyzed and the priority actions to implement the requirements of the Water Framework Directive in Ukraine were proposed.