Over last decade an increasing interest for antioxidants in foods has arisen. The healthy properties of antioxidants related to the prevention of degenerative diseases are the main cause of this boom. An antioxidant prevents the oxidation process, the initial step of development of degenerative diseases, cancer and many others. Literature encompasses analytical methodology development to assess antioxidant properties of foods and beverages. The screening of antioxidant activity of foodstuffs is the subject of a large number of articles. Special interest has been addressed to wine, tea and chocolate. However, the crucial key in the prevention of disease is the action these antioxidants exert after their consumption. Studies involving human subjects are scarce due to the requirements of availability of volunteers and conditions to test are limited. This review summarizes data related to in vitro antioxidant activity of foods, emphasizing the main role of phenolic compounds. A critical comparison is realized between the biological significance of these values and the biological significance of in vivo measurements. In addition, the Plasma Antioxidant Capacity is evaluated and selected as biomarker for in vivo antioxidant status of human organism. In a second part, data collected from different intervention studies performed up to date are compiled and discussed. This review summarized data related to in vitro antioxidant activity of foods, emphasizing the main role of phenolic compounds. A critical comparison is realized between the biological significance of these values and the biological significance of in vivo measurements. In addition, the Plasma Antioxidant Capacity is evaluated and selected as biomarker for in vivo antioxidant status of human organism. In a second part, data collected from different intervention studies performed up to date are compiled and discussed. The original contribution of this work is to compile data of Plasma Antioxidant Capacity after dietetic intervention studies taking into account the portion of food ingested. In addition, we calculated the antioxidant compounds content (phenolic content, ascorbic acid, vitamin E and carotenoids) contained in each food ingested to evaluate better their impact in Plasma Antioxidant Capacity. Intervention studies are grouped by the length of intervention and type of food ingested. Results reported in literature reveal that the increment in Plasma Antioxidant Capacity largely depends on analytical method used.