Adverse reactions to foods and food additives are common problems. True food allergy involving an immunological mechanism is less common; these allergies affect approximately 10% of the United States population. Most food allergies are immunoglobulin E-mediated; systemic anaphylaxis is the most serious complication. Diagnosis is based on history and physical examination. In vivo diagnostic studies involve skin testing and food challenges (blinded and unblinded). In vitro testing (most commonly radioallergosorbent test [RAST]) is widely used. Treatment is best accomplished by identifying and avoiding the offending allergen.