Assessment of optic disk size is an important, but often overlooked, component of the diagnostic evaluation for glaucoma. Measured values of optic disk size vary with the measurement technique utilized. Available methods for disk size measurement and their respective strengths and limitations will be discussed. Further, actual disk size varies with race and possibly other demographic characteristics. Disk size is also associated with variation of specific anatomical structures of the optic nerve head and the retinal nerve fiber layer. These disk size- dependent variations may influence the susceptibility to glaucoma or the likelihood of glaucoma diagnosis. This manuscript reviews the published evidence relating to disk size and glaucoma.