Despite its critical importance to our daily life, the most common measurement of visual function, visual acuity, is a relatively crude and narrow one testing only a small portion of the broad range of visual functions. Visual acuity is the measurement of the ability to discriminate two stimuli separated in space at high contrast relative to the background. Clinically, this is measured by asking the subject to discriminate letters of known visual angle. The visual acuity is represented as the reciprocal of the minimal angle of resolution (the smallest letters resolved) at a given distance and at high contrast. Other measurements of visual acuity also exist, including Vernier acuity. Newer charts, such as the ETDRS chart, use letters of equal recognition difficulty and use the log of the minimal angle of resolution; these charts have significant advantages over the old Snellen-type charts. This article reviews visual measurements in children and in patients with low vision, and it reviews factors affecting visual acuity, such as pupil size, refractive error, media opacities, and pharmacologic agents.