The present study is devoted to theoretical questions of optimization of integrated assessment of the composition and properties of drinking water with the use of the Water Quality Index (WQI) and considering in it all 4 criteria for its hygienic quality-sanitary-toxicological, microbiological, radiation and organoleptic. There is presented a sequence of the analysis of benchmark data of the laboratory study of drinking water, including the selection of priority indices, their distribution into 4 groups according to hygienic criteria, calculations the ratios of real values (C) of indices to their hygiene MPC and the final calculation of the WQI. There is emphasized the importance of classes of hazard of substances, and the need for the special attention to the substances-carcinogens in the integrated assessment of water quality. To overcome the non-equivalence of contributions to the assessment of water quality factors, measured in different units, often disparated in their effect on human health, there are used the principles of combined action at levels below the MCL:C/MPC indices of performance of the unidirectional action are summed (e.g. carcinogenic substances), from indices of the independent action there are selected the most significant ones with the highest values of C/MPC, besides that there are also used counterbalancing factors K determined accordingly to Delphi method, with a maximum values of 5 for carcinogens and the minimum value of 1 for the substances affecting the organoleptic properties ofwater. There is presented the scheme of the final calculation of the value of WQI.