The diagnosis of a contact allergy requires several important and essential steps, because the failure to recognize a contact allergy can occur in any of the various stages of the contact allergy investigation. If the results of the skin tests are negative for a patient for whom a diagnosis of allergic contact dermatitis has been proposed, one has to go back to the beginning, that is, with a thoroughgoing anamnesis of the patient (perhaps with a visit to his or her environment). The assumed allergens must be retested (perhaps in another concentration, with another vehicle, or with another testing method), and additional allergens must be tested. The patient can also be asked to keep a journal, in the hope that a correlation can be discerned between exposure to a substance and the occurrence of the skin problems. Once an allergen has been identified, it is the dermatologist's task to provide specific advice about the products that have to be avoided, or about the products that can safely be used, because subjects sensitive to specific ingredients must avoid products containing them.