Onychomycoses represent the most frequently seen nail diseases and are the most difficult to treat of all skin mycoses. They are rare in children and increase in incidence with age. Most cases are caused by dermatophytes, in particular by Trichophyton rubrum, less frequently by T mentagrophytes and Epidermophyton floccosum. Molds may secondarily infect nails already diseased; however, some are probably capable of primary invasion of nail tissues. Yeasts, particularly Candida albicans, are mainly isolated from fingernails in chronic paronychia and onycholysis, and from nails in chronic mucocutaneous candidosis. Mixed infections by dermatophytes, molds, and/or yeasts are not uncommon. Probably, most fungi cannot infect a healthy nail organ, and only predisposing factors such as impaired blood circulation, peripheral neuropathy, diabetes mellitus, damage from repeated minor trauma, and limited immune defects as well as AIDS make the nail susceptible to fungal infection. Most onychomycoses are secondary to a mycosis of the adjacent skin. Distallateral subungual onychomycosis starts at the hyponychium spreading proximally to the nail bed and matrix. In proximal subungual onychomycosis, the fungus infects the cuticle and eponychium to reach the matrix where it becomes enclosed into the nail plate substance. Total dystrophic onychomycosis may result from either form or develop in chronic mucocutaneous candidosis. Superficial white onychomycosis is commonly a culture of T mentagrophytes on the surface of a toenail. Mycotic paronychia and onycholysis are usually due to C albicans. Clinically, onychomycoses have to be differentiated from noninfectious onychodystrophy, nail psoriasis, lichen planus unguium, and chronic nail eczema. Despite a considerable number of effective antifungal drugs, treatment has remained difficult because the predisposing factors are usually not amendable to therapy.