Surgical treatment of glaucoma is usually reserved for serious cases which cannot be controlled by other means such as topical medication or laser. Surgery can be performed in most types of glaucoma in order to reduce the pressure inside the eye which if left uncontrolled can irreversibly damage the optic nerve leading to loss of sight. There are a number of variants of drainage surgery but the most commonly performed procedure is trabeculectomy in which a guarded channel is fashioned through the wall of the eye under the upper lid and the fluid is allowed to collect under the conjunctiva (the membrane lining the eyeball) to form a drainage bleb. Scarring during the healing process can cause this channel to close and the operation to fail with a rise in pressure. Mitomycin C is a powerful agent which prevents scarring by inhibiting the multiplication of cells which produce scar tissue. This review asks whether there is evidence that its use during the initial stages of surgery to prevent the excessive conjunctival scarring reduces the risk of failure of the operation. Three types of patient were included: those at high risk of failure because of previous failed surgery or other complications, those having combined cataract and glaucoma surgery and those having primary trabeculectomy - an operation for the first time for their glaucoma. The review found evidence that Mitomycin C reduces the risk of surgical failure in both high risk and primary surgery but no evidence on combined cataract and glaucoma surgery. But the risk of adverse effects including an increased risk of cataracts (not in the combined group) was also noted. There were only a few studies on each category of patients and most were of only poor or moderate quality.