The present history is most important to pick out an allergen causative of his/her allergy. Skin tests or tests for serum are available for the detection of allergen-specific IgE. Skin tests include prick test and intradermal test. The former should be preceded the latter to avoid systemic allergic adverse reactions with the latter. Each test for blood or skin has both merits and demerits. Sensitivity of skin test is better than blood test. Skin patch test can detect delayed type allergy, additionally cross reacting antigens should be concerned. The immunologic mechanism of drug allergy has not yet been elucidated enough.