Binocular summation was evaluated for contrast detection and discrimination. Monocular and binocular forced-choice psychometric functions were measured for the detection of 0.5-c/deg sine-wave gratings presented alone (simple detection), or superimposed on identical background gratings (discrimination). The dependence of detectability d' on signal contrast C could be described by: d' = (C/C')n. C' is threshold contrast, and n is an index of the steepness of the psychometric function. n was near 2 for simple detection, near 1 for discrimination, and was approximately the same for monocular and binocular viewing. Monocular thresholds were about 1.5 times binocular thresholds for detection, but the ratio dropped for suprathreshold discrimination. These results reveal a dependence of binocular summation on background contrast. For simple detection, binocular detectabilities were at least twice monocular detectabilities . For contrast discrimination, the amount of binocular summation decreased. For a 25%-contrast background, there was little or no binocular summation. It is concluded that binocular contrast summation decreases as background contrast rises.