Onychomycosis is an infection of the finger-and/or toenails by fungal microorganisms. If untreated, the process advances and destroys the nail plate. It may spread to involve the skin and does not heal spontaneously. There are different clinical presentations of onychomycosis which vary with the nature of the fungus and how it invades the nail unit. These different clinical forms require different therapeutic approaches. The successful treatment of onychomycosis requires special knowledge of the various clinical presentations, of the differential diagnosis and of recent advances in medical mycology. Therefore onychomycosis is best treated by dermatologists.