The difference in point of view between the limnologist and the public on the trophic level of a water body is discussed in relation to actions promoted by administrators. Numerical and probabilistic classification, models, "trophic indices" and biological indicators for assessing the trophic degree of lakes are briefly illustrated. The contribution of the OECD's "Eutrophication programme" (1971-1979) to the assessment of the trophic degree of lakes is relevant. In agreement with Vollenweider's model, this programme adopted as a guide-line the relationship between phosphorus load from the watershed and the phosphorus concentration in the lake water and between this concentration and that of phytoplankton, expressed as chlorophyll concentration. These relationships are statistically significant but a certain number of lakes deviate from them. These deviations and their causes are considered ; for example, nutrient release from sediments, short hydrological relention time, high mineral turbidity, excessive development of macrophytes, non available forms of phosphorus, high predation pressure by zooplankton. The opportunity to take into consideration the influence of toxic pollutants in eutrophicated water bodies is emphasized. To compare the trophic degree of lakes laying at different latitudes, the pattern of solar radiation and temperature should be considered, in addition to nutrient loading. As a case study to show the difficulties involved in assessing the trophic degree of a lake and identifying the causes of its evolution the results from a study carried out from 1969 to 1980 on the small alpine lake Monate (Northern Italy) are discussed.