Thus, the aim of the present study was firstly to examine the antioxidant effects of stem extracts from three Greek grape (Vitis vinifera) varieties (i.e., Mandilaria, Mavrotragano and Moshomavro) at a cellular level, particularly in human umbilical vein endothelial EA.hy926 and C2C12 muscle cells. Moreover, since the polyphenolic composition has been shown to affect the antioxidant activity of wine and grape seed extracts (26,27), we also investigated whether the polyphenolic composition of grape stem extracts affects their antioxidant capacity. Grape stem extracts may be used as antioxidant food supplements for reducing the oxidative stress-induced detrimental effects on human health, and thus the understanding of the mechanisms through which they exert their bioactivities is of particular importance.