"Allergic" manifestations are not necessarily of allergic origin; they can also be produced by non-allergic factors and their true allergic nature must, therefore, be proved. A detailed personal history and the presence of eosinophilia (in the blood or secretions) are suggestive evidence which has to be confirmed by the identification of specific antibodies of the immunoglobulin-E type. Criteria for arriving at the diagnosis "allergy" are (a) demonstration of antibodies by skin and mucous membrane tests with allergen extracts; (b) demonstration of the activity of the identified allergen by challenge tests on the reacting organ (conjunctiva, nose, bronchi); (c) in-vitro demonstration of the allergen-specific immunoglobulin-E by means of the radio-allergo-sorbent test. Details for the performance of the various tests are given and the interpretation and significance of the findings are discussed.