Investigations of the rate at which sebum is produced in man have played an important part, not only in the understanding of the physiology of the sebaceous gland and the pathogenesis of acne vulgaris, but also in the search for  effective treatments.  The purpose of this chapter is to describe critically the methods which have been used to measure the rate of sebum production and delivery, their interpretation and the modulating effect of certain physiological and pathological situations on sebum production.