This review addresses the problems of defining severity of psoriasis. Concepts of severity depend on the timescale perspective from which judgement is made. Measurement needs to include assessment of signs, impact on the patient's life and the history of the disease. The concept of severity in relationship to quality of life measurement scores has been defined, so it is now possible to postulate a standard, easily remembered concept to help define 'severe psoriasis' in the clinic. The proposed Rule of Tens for current severe psoriasis from the clinician's viewpoint is: 'Current Severe Psoriasis = Body Surface Area involved > 10% or Psoriasis Area and Severity Index score > 10 or Dermatology Life Quality Index score > 10'.