Over the last decades the prevalent research on stevia has been focused on its steviol glycosides. However, apart from the sweet glycosides, stevia leaves contain antioxidants, mainly chlorogenic acids but also flavonoids. Their antioxidant capacity might be useful for food preservation by preventing oxidative degradation. The antioxidant capacity can be measured in vitro by various assays that are presented in this chapter, of which only the oxygen radical absorbance capacity assay is standardised. Stevia leaves seem to be among the foods with the highest antioxidant capacity. Its antioxidant relevance for human health is questionable. For that, some evidence was obtained, mainly by bioassays, giving promising results. Nevertheless, further in vivo surveys are needed to substantiate the putative protective role of stevia against oxidative stress in humans. Stevia leaves gained attention due to their sweet taste caused by the steviol glycosides. Like all plants, stevia contains a variety of secondary metabolites namely polyphenols as summarised in Chapter 5 of this book. A common characteristic of most polyphenols is their antioxidant capacity or activity. We prefer the term “capacity”, because “activity” implies an acting role. This chapter focuses on the antioxidant properties of the stevia plant and how it is measured. A thorough review of the literature is provided to illustrate this less studied property of the crop.