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Onychomycosis is a fungal infection of the nails that causes discoloration, thickening, and separation from the nail bed. Onychomycosis occurs in 10% of the general population, 20% of persons older than 60 years, and 50% of those older than 70 years. It is caused by a variety of organisms, but most cases are caused by dermatophytes. Accurate diagnosis involves physical and microscopic examination and culture. Histologic evaluation using periodic acid-Schiff staining increases sensitivity for detecting infection. Treatment is aimed at eradication of the causative organism and return to a normal appearance of the nail. Systemic antifungals are the most effective treatment, with meta-analyses showing mycotic cure rates of 76% for terbinafine, 63% for itraconazole with pulse dosing, 59% for itraconazole with continuous dosing, and 48% for fluconazole. Concomitant nail debridement further increases cure rates. Topical therapy with ciclopirox is less effective; it has a failure rate exceeding 60%. Several nonprescription treatments have also been evaluated. Laser and photodynamic therapies show promise based on in-vitro evaluation, but more clinical studies are needed. Despite treatment, the recurrence rate of onychomycosis is 10% to 50% as a result of reinfection or lack of mycotic cure.