Glass breaking in compartment fires is an important practical problem since a window acts as a wall before breaking and as a vent after breaking. If sufficient excess pyrolyzates have accumulated in the hot layer, this sudden geometric change can lead to back draft and flashover. As Emmons explained at the first Symposium, windows break in fires due to thermal stress from the differential heating of the central portion and the shaded edge. The focus of this paper is on quantifying the connection between the compartment fire and the glass temperature to predict the window breaking time. tb. Techniques are presented for accurately calculating the history of the central glass temperature profile, T(x, t), for any fire exposure. Two-dimensional temperature histories, T(x, y, t), where x is depth and y is toward the center, and mean stress histories, ?zz, (y, t), are also calculated. It is determined here that breaking occurs when the mean glass temperature difference is ?T = (?b/E ß)g , where ?/E is the maximum glass tensile strain, ß is the thermal coefficient of linear expansion and g is a geometry factor of order one. Calculations suggest that the edge remains at its initial temperature, Ti, so that ?T = T (tb) - Ti, when the shading is large, s/L>=2 , and the heating is fast, aib/s2=<1, where L is the glass thickness, s is the shaded edge width and a is the glass thermal diffusivity.