OCT or Optical Coherence Tomography of the nerve fibre and ganglion cell layers have become the gold-standard in the assessment of the optic nerve in glaucoma. There are a number of devices on the market with this lecture focusing on the Cirrus from Zeiss. In order to intepret the output from the machine, it is important to know how the machine generates the data, the comparative database, the various segments of the report and how variations in the anatomy of the optic nerve can influence the scan.  Although it has become an integral part of a comprehensive glaucoma consultation, it must be taken in context with other patient findings including the intraocular pressure, optic nerve examination and visual fields.