This is a common worldwide disease, affecting 10-20% of people over a lifetime in one form or another. It is characterized by transient itchy superficial pink or pale skin swellings (weals) that may be accompanied by deeper swellings of the skin, mouth or genital region (angioedema) in up to 50% of patients. The swellings come and go quickly without leaving a mark. Patients with severe urticaria may also feel unwell with non-specific symptoms that include tiredness, feeling too hot, aching or indigestion. Weals and angioedema are common features of anaphylaxis and some types of urticaria may, very rarely, progress to anaphylaxis. Most patients with urticaria become better quickly but a few will continue to develop episodes of swelling for months or years. Urticaria is called acute if it goes away within 6 weeks and chronic if it continues regularly beyond this. Histamine is the main mediator of urticaria. It is a natural chemical that is normally contained in granules found in specialized cells, called mast cells, in the skin and other parts of the body. They release histamine more easily than they should in urticaria so an important part of the initial assessment is to try and find out what causes this. It is often thought that urticaria is due to an allergy but, in reality, it is uncommon to find an allergic cause for the release of histamine in acute urticaria and almost never in chronic urticaria.