In this chapter the theoretical reasons were outlined and clinical data summarized as to why spatial contrast measurements can reveal visual losses that are not uncovered by testing visual acuity, no matter which optotype one uses or how carefully the measurement is made. Spatial contrast sensitivity measurements may disclose different types of contrast losses in patients with different lesions but with identical visual acuity. The relationship of different types of spatial contrast sensitivity losses (plotted as visuograms) to specific location or cause of lesions is not yet clear. Patients with glaucoma show losses that occur infrequently in other types of eye or visual pathway diseases, but the specificity of the typical contrast loss needs confirmation. The definite clinical value of contrast sensitivity measurements is that they can identify incipient abnormalities in the visual pathways that subserve foveal vision. In addition to this definite diagnostic application, using sinusoidal gratings as stimuli for both contrast sensitivity and for VEP measurements is useful in a research-oriented clinical testing facility.