Onychomycosis describes a chronic fungal infection of the nails most frequently caused by dermatophytes, primarily Trichophyton rubrum. In addition, yeasts (e. g. Candida parapsilosis), more rarely molds (Scopulariopsis brevicaulis), play a role as causative agents of onychomycosis. However, in every case it has to be decided if these yeasts and molds are contaminants, or if they are growing secondarily on pathological altered nails. The point prevalence of onychomycosis in Germany is 12.4%, as demonstrated within the "Foot-Check-Study", which was a part of the European Achilles project. Although, onychomycosis is rarely diagnosed in children and teens, now an increase of fungal nail infections has been observed in childhood. More and more, diabetes mellitus becomes important as significant disposing factor both for tinea pedis and onychomycosis. By implication, the onychomycosis represents an independent and important predictor for development of diabetic foot syndrome and foot ulcer. When considering onychomycosis, a number of infectious and non-infectious nail changes must be excluded. While psoriasis of the nails does not represent a specific risk factor for onychomycosis, yeasts and molds are increasing isolated from patients with psoriatic nail involvement. In most cases this represents secondary growth of fungi on psoriatic nails. Recently, stigmatization and impairment of quality of life due to the onychomycosis has been proven.