Adverse food reactions may be secondary to food allergy (hypersensitivity) or food intolerance. The clinical manifestations of food allergies depend on the target organ affected. Gastrointestinal, respiratory, and cutaneous symptoms are the most common of the clinical responses. The medical history, physical examination, and various in vivo or in vitro tests are useful in the diagnostic evaluation. Double-blind, placebo-controlled food challenges are the standard for diagnosis of food allergies. Presumptive diagnosis of food allergy based on patient history and on results of skin test or radioallergosorbent test is no longer acceptable, except in cases of severe anaphylaxis after an isolated ingestion of a specific food. Unless the physician provides an unequivocal diagnosis of food allergy, people will continue to alter their eating habits on the basis of misconceptions of food allergy.