Human organisms have many defence mechanisms able to neutralise the harmful effects of the reactive species of oxygen. Antioxidants play an important role in reducing the oxidative damage to the human organism. Carotenoids are among the strongest antioxidants. They have 11 coupled double bonds, so they can be classified as polyisoprenoids, show low polarity and can occur in acyclic, monocyclic or bicyclic forms. The carotenoids of the strongest antioxidant properties are lycopene, lutein, astaxanthin and β-carotene. Carotenoids with strong antioxidant properties have found wide application in medical, pharmaceutical and cosmetic industries. These compounds are highly active against both reactive oxygen species and free radicals. Comparing β-carotene, astaxanthin and lycopene with other antioxidants (e.g. vitamin C and E), it can be concluded that these compounds have higher antioxidant activity, e.g. against singlet oxygen. Astaxanthin is a stronger antioxidant compared to β-carotene, vitamin E and vitamin C, respectively 54, 14 and 65 times. Carotenoids have a salutary effect on our body, making it more resistant and strong to fight chronic diseases. The purpose of this article is to review the literature concerning free radicals and their adverse effects on the human body and carotenoids, as strong, natural antioxidants.