This chapter builds on the concepts introduced in Chapter 1 which, apart from the bibliometrics, concluded with an elaboration of the meaning of antioxidant activity. Chapter 3 begins with an examination of the reasons for measuring antioxidant activity rather than concentration. After all it is the concentration of most analytes that is measured in samples rather than activity. Why this difference? The measurement of antioxidant concentration is an inadequate measure of activity in most instances. General issues associated with measurement of antioxidant activity are discussed, and the methods used to measure antioxidant activity are outlined and classified. Antioxidant activity can only be expressed in relation to an oxidizable matrix or substrate so the different sample matrices are examined.