Patients with urticaria make up a large proportion of the referrals to allergy clinics. There are many causes of urticaria and it is the clinical history which is most important when attempting to identify potential causes; however, urticaria is very often idiopathic. In a small minority of patients urticaria may be a symptom of a serious underlying medical illness or the allergic symptoms may progress to cause systemic reactions, and it is important to identify these patients and to remember that severe urticaria is a distressing and disabling condition. This review will discuss classification, investigation and treatment of urticaria and will consider some of the more unusual types of urticaria that may be encountered in the out-patient clinic.