Although dermatophytoses can often be diagnosed clinically, the identification of the dermatophyte species causing the disease gives additional relevant information. Species identification is necessary to survey the current epidemiologic situation, and it is also needed in each individual case to trace back the path of infection, thereby to avoid reinfection, and to optimize the therapy. Classical methods to differentiate dermatophytes are based on the assessment of their macroscopic and microscopic morphology and on the observation of physiologic properties. For scientific taxonomic purposes mating experiments are indispensable. In recent years, DNA analysis has been refined to make a genetic-based differentiation of dermatophyte species possible, although such methods are not available for routine practice. No elaborate technical equipment is needed to differentiate dermatophytes in the office. The investigator's mycological expertise, however, is of crucial importance. Mycological courses and dermato-mycological quality control are appropriate means to ensure that dermatologists retain their competence in treating patients with dermatophytoses.