Review of 14 years of data concerning the geographic and hydrographic characteristics of Léman show that this lake is in the process of eutrophication: nutrient levels are increasing, oxygen levels are decreasing in the deep waters, and the algal flora has changed. The effect of the current warming of the climate on its evolution is taken into consideration.Since 1900 the fishing regulations of the Swiss and French have been dissimilar. Excessively small size limits and at certain times the abusive use of nets have repeatedly led to a reduction of the most valued species.French catch data, covering more than 70 years, reveal fluctuations which reflect biological disturbances due primarily to overexploitation and also to eutrophication. Landings have increased but value has decreased.Numerous introductions have been made purposely or accidentally. Certain of these species have had deleterious effects (sticklebacks) and some beneficial (introductions of a new population of coregonines after the extinction of the indigenous species).Some aspects of the biology of the most valued salmonids (Salmo trutta, Salmo gairdneri, Salvelinus alpinus) have been shown as well as the results of attempts to repopulate them using pisciculture and marking. Plantings of rainbow trout along the shoreline during summer gave best results (percentage return and growth being most important), although their reproduction seems to be doubtful.Overexploitation of coregonines followed by that of perch (Perca fluviatilis) has left roach (Rutilus rutilus) to inhabit all the central zone of the lake. This valueless species is favored by eutrophication and is thus able to utilize plankton without undergoing competition from coregonines and young perch, or predation by larger perch. No fishing profits are derived from the invasion of the lake by roach.