We interpret oxygen and hydrogen isotope data for water samples from Lake Baikal, Siberia, its tributaries and other local rivers, and local precipitation in terms of the known water budget for the modem lake in order to gain insight into past limnological and climatic processes that influenced the lake. Lake Baikal is remarkably uniform in its isotopic composition (??18O = -15.8 ?? 0.2???; ??D = -123 ?? 2???) and lies slightly to the right of the global meteoric water line, which suggests significant evaporation. Water is supplied to the lake by over 300 rivers and streams. The oxygen isotope values (??18O) of the rivers in the Baikal catchment range from -13.4 to -21.2???. The hydrogen isotope values (??D) for the same area range from -103 to -156???. Both these ranges generally conform to the global meteoric water line. The weighted average isotopic composition of input to the lake (rivers plus precipitation) is -15.2??? for ??18O and -116??? for ??D, values higher than those of the modem lake. Therefore, the isotopic composition of the modem lake cannot be related to the modem input through simple evaporation. Instead, modeling of the isotopic mass balance of the lake suggests that inputs (precipitation and influx from rivers) and outputs (evaporation and outflow) are not at a steady-state equilibrium under current climate conditions. We found previous input to the lake had lower ??18O and ??D values than modem input, which reflects cooler climates in the past compared with modern conditions. Under constant climate conditions, steady-state conditions are not expected to be reached by the lake for at least 700 yr because of its large size and the long residence time of water in the lake.