Forced-choice psychometric functions were determined for the detection of sinewave gratings and contrast discrimination of near-threshold gratings at spatial frequencies of 0.5, 2 and 8 c/deg. Detection psychometric functions all had the same S-shaped form. Discrimination functions were almost linear except at the upper end. Both sets of data can be described well by a detection model with a positively accelerating relation between contrast and mean decision variable and a differencing decision rule. Results of a paired comparisons experiment were consistent with the model and indicate that decision variable variance is nearly constant over the range of contrasts used in these experiments. The implications of these results for several models of contrast detection and discrimination are considered.