Clinical examination of the optic nerve and achromatic automated perimetry is the gold standard for the management of glaucoma. However, there is an increasing need for an objective evaluation of the optic nerve structure, particularly for preperimetric glaucoma. Optical coherence tomography (OCT) is a noninvasive tool that measures retinal nerve fiber layer (RNFL) thickness based on its optical properties. Computer image processing algorithms estimate NFL thickness from circumpapillary OCT images that are acquired in cylindrical sections surrounding the optic disc. Average values of NFL thickness can be calculated in the four quadrants or the 12 o'clock position sectors around the optic disc. The mean NFL thickness around the entire disc can also be calculated. NFL thickness values may be compared to a normative database. Although this technique offers objectivity, rapidity, and reproducibility, it is largely influenced by the variability of optic disc size and the number of nerve fibers among individuals. At present, OCT is a good instrument to diagnose early glaucoma, but cannot be used to exclude it. New technologies, like spectral domain and ultra-high resolution, which are already available, will overcome the limitations of OCT.