Foods can induce adverse reactions by a variety of mechanisms. An understanding of the characteristic signs and symptoms and the related mechanisms of adverse food reactions allows the clinician to efficiently diagnose and treat patients. Adverse reactions to foods can be classified based on whether there is a nonimmunologic or immunologic basis for symptoms. Food intolerance, or a nonimmunologic reaction, includes a range of responses to foods that result primarily from an individual’s intrinsic inability to metabolize a component of the food,e.g., lactose sugar in dairy products. Other nonimmunologic adverse reactions may be attributed to food toxins or pharmacologic properties pharmacologic properties of foods themselves. Immunologic adverse reactions, in contrast, involve immune responses to food and are termed food allergy. Food allergy may further be categorized based on the underlying immunopathophysiology as immunoglobulin E (IgE) mediated, non‐IgE mediated, or cell mediated. Some chronic allergic responses involve a combination of immune mechanisms. This review provides a general classification system for adverse food reactions and describes specific conditions.