Confirmed adverse reactions to foods may be caused by toxic, enzymatic, pharmacological, "pseudoallergic" or allergic mechanisms. True food allergies are mostly IgE-mediated and directed against one or only a few food proteins. They appear typically as eczema and gastrointestinal symptoms (vomiting, diarrhoea, abdominal cramps) among infants and as oral allergy syndrome, urticaria/angioedema, rhinoconjunctivitis or anaphylaxis among adults. The majority of food allergies among adults is caused by cross-reactivity of IgE against inhalative allergens also reacting with food proteins. This must be considered in investigations by skin-prick testing and/or specific IgE measurement, since the sensitivity of these tests for inhalative allergens is higher than for food proteins. The most frequent differential diagnoses of true allergies are pseudoallergic reactions to food additives or pharmacological reactions to biogenic amines. The diagnosis of these reactions can usually be based on the history and course under a corresponding diet. In clinical practice additional investigations by double-blind placebo-controlled food challenges are rarely required. A positive challenge test demonstrates only the cause-and-effect relationship of the foods and the patient's symptoms but does not demonstrate the underlying mechanism. The therapy of food intolerance is a corresponding diet. This requires a careful diagnosis and identification of the causative foods.