Phenolic acids are compounds, which vary in their chemical structure and properties; they are very common in the world of plants. Recently, phenolic acids become more and more popular as diet components owing to their beneficial impact on human health. In this paper, an attempt was made to recapitulate all the data referring to biological activity of phenolic acids, their biosynthesis, and their occurrence both in the world of plants and in the plant-based food. In particular, the following issues were studied: their antioxidant activity, a mechanism of their antioxidant actions, and the relationship between the chemical structure and antioxidant activity. The medicinal and pharmacological properties of phenolic acids are mainly connected with their antioxidant activity. It was also attempted to explain their metabolism in the human gastrointestinal tracts, and, consequently, their bioavailability.