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The diagnosis of drug allergy is essentially based on a detailed anamnesis, involving the doctors who first treated the patient, and skin testing (prick, intradermal and epicutaneous / patch tests). In the allergological practice / clinic, provocation tests with the presumed trigger are only carried out if the indication is very clear (see articles in this issue on drug allergy children, allergies to betalactam and other antibiotics as well as analgesic intolerance). The provocation with a probably tolerable alternative is in the foreground. Unfortunately, the skin tests of certain drug groups have a low sensitivity even under optimal conditions, but very good specificity. Accordingly, positive skin tests are mostly relevant, but negative skin tests cannot rule out an allergy. In recent years, it has therefore proved successful to carry out supplementary laboratory tests in the clarification of drug allergies. The serological tests (IgE) are of little help. In contrast, the test forms based on the analysis of leukocytes (basophil activation test, BAT, and lymphocyte transformation test, LTT) have gained in importance and complement the diagnostic repertoire. In the combination of all test methods (skin test, LTT, BAT, sometimes provocation test) the trigger of a drug allergy can be defined in a good 70 % of cases and in most cases a safe therapeutic alternative can be found. In the following, we will discuss the importance of laboratory diagnostics in drug allergy.