In some patients with neural or ophthalmological lesions, normal visual acuity co-exists with a significantly diminished ability to see larger objects. The converse may also occur. Based upon many years of academic research and some years of clinical application, spatial contrast sensitivity function appears to give the most general description available for the basic aspects of spatial vision performance. This review surveys the main theoretical and experimental works in neurophysiology and psychophysiology which established the significance of contrast sensitivity.