The diagnosis of onychomycosis should be made clinically and mycologically: clinically, by one of seven subtypes of onychomycosis, and mycologically, by evidence of dermatophytes or verified presence of molds and/or yeasts. Dermatophytes are usually considered as pathogens, whereas non-dermatophyte molds and yeasts are saprophytes. Basic anamnesis and close inspection should be performed to eliminate combined diseases (e.g., onychomycosis and trauma). The gold standard treatment for onychomycosis is basically systemic. Combination with topical agents, such as nail lacquer and/or chemical nail avulsion, produces better results than systemic treatment alone. Topical treatment as monotherapy is not efficient, excluding minor cases. Terbinafine is superior to itraconazole for dermatophyte onychomycosis. Evaluation of the outcome of clinical cure, mycological cure and total cure should be based on the well-defined worldwide criteria; otherwise, comparison of results is impossible due to lack of uniformity in different studies. In case of treatment failure, the reasons for each failure should be carefully considered.