Winterkill is a common phenomenon in many eutrophic lakes subject to prolonged ice and snow cover. These conditions can lead to greatly reduced oxygen absorption and generation under ice‐cover, compared with oxygen consumption, and to dissolved oxygen (DO) depletions below concentrations required for fish survival. DO requirements of fishes under ice‐cover are not well defined, but are much reduced compared with summer conditions, with many fishes surviving at DO concentrations <1.0 mg/1 at water temperatures of 0 to 4°C. A lower DO limit of 2 mg/1 for gamefish has been suggested. Typically, as under‐ice DO concentrations become progressively depleted, the more valued gamefish succumb first, leaving less desirable fishes; and, in extreme cases, no fish survive. Frequencies, causes, and preventions of winterkill have been most thoroughly studied in Michigan, Minnesota, Wisconsin, and the Great Plains region of the U.S. and Canada. Long‐term records from these areas indicate that winterkill frequency is related to duration and intensity of winter weather conditions; but some lakes experience winterkill almost every year regardless of the weather.