The book provides general provisions for assessing water quality by the detailed method and the method of complex indices, showing the features of assessing water quality according to the norms of the EU countries. On the example of the Danube and Dniester rivers, the chronological variability of the values of water quality indicators is considered. The trends of indicators are approximated and the parameters of their distribution laws are determined. It was found that when approximating the distribution of indicators it is more convenient to use the lognormal law, that the time trend of indicators better reflects the exponential dependence, while the lognormal distribution of the indicator normalized along the trend line can formally be considered one-parameter: one of the parameters (the average value of the logarithms of the normalized series) becomes zero. It is shown that for the previous period of time and in the future, when standardizing discharges of pollutants with wastewater, the assessment of water quality in control sections of water bodies will meet the requirements of the EU countries' norms for the frequency of exceeding the MPC.