Onychomycosis is an increasingly common fungal infection of the nail, which has traditionally been difficult to diagnose and treat and has physical and psychological consequences for the patient. Onychomycosis can be caused by dermatophytes, nondermatophytic filamentous fungi, and yeasts. The relative percentages of cases due to these etiologic agents vary with geographic location; however, in the United States, dermatophytes are the most common pathogens. Toenails are affected four times as often as fingernails. Microscopy and culture are the diagnostic "gold standards" for onychomycosis, although biopsy of the nail may be required to obtain a definitive diagnosis when conditions that mimic onychomycosis, such as psoriasis, are suspected. The treatment of onychomycosis includes a combination of topical therapy, surgical or chemical nail avulsion, and systemic therapy. The new generation of systemic agents (itraconazole, fluconazole, and terbinafine) is associated with a higher cure rate and shorter courses of treatment than are the older systemic antifungal drugs (i.e., griseofulvin and ketoconazole); these characteristics have sparked new interest in onychomycosis. Of these newer antifungals, itraconazole and terbinafine are the only agents currently approved by the U.S. Food and Drug Administration for the treatment of onychomycosis.