Coumarins and flavonoids are heterocyclic molecules that have been associated with beneficial effects on human health, such as reducing the risk of cancer, diabetes, cardiovascular and brain diseases. These effects are thought to be related to the radical scavenging effect, due to their antioxidant activities, along with other possible mechanisms, such as anti-inflammatory properties and interaction with several enzymes. Over the past two decades, there have been an increasing number of publications on coumarins and flavonoids, which demonstrate the importance of understanding the chemistry behind the antioxidant activities of both natural and synthesized compounds, considering the benefits from their dietary ingestion as well as pharmacological use. This work aims to review the antioxidant effects of coumarin and flavonoid molecules in humans and the structural aspects that contribute to these effects.