Lake Sapanca is the major water resource supplying drinking water as well as water for industrial and agricultural uses for one of the more industrialized areas of Turkey. Currently, the sustainability of its beneficial use is an issue of concern. Initially the water quality in the lake was classified according to the Turkish Regulations. With this conventional approach, a detailed water quality evaluation could not be achieved due to certain drawbacks, and this required the search for a new approach. Therefore, a more detailed classification was accomplished by taking German Technical Standards as a basis that would lead to more usable and understandable classification necessary to promote a rational management strategy. Both classifications dictate that classes 1 and 2 may be used for water supply effectively. The detailed classification indicates that at present water quality in the lake is overall of first class; however, it is tending towards Class 2, with the lake in a state of transition from oligotrophy to mesotrophy and the most critical issues being nutrient concentrations in the lake and land based nutrient sources. Copyright (C) 1996 IAWQ.