**2. Classification**

The current classification of urticaria is based upon the clinical course of the disease. Subtypes include spontaneous urticaria, physical urticarias and other urticaria types [2]. Spontaneous urticaria can either be acute lasting less than 6 weeks or chronic with a duration of more than 6 weeks. This arbitrary division for chronicity has been made as the etiologies and thus the clinical evaluation of acute and chronic urticaria vary considerably [3]. The physical and other urticaria types are elicited by external stimuli, such as heat, cold, pressure, vibration, friction, sunlight, water, etc. [6]. These two subtypes of urticaria are beyond the scope of this chapter.
