Epidemiological and clinical studies have proved the beneficial effect of bioactive compounds of vegetables and medicinal plants used in phytotherapy on prevention of several diseases. Beyond the well-known natural antioxidants, such as vitamins, other natural substances can function as antioxidants. In 1992 the Saas Fee Declaration was published by leader nutritionist and biochemists underlining the importance of prevention in health maintenance based on scientific studies with natural compounds. According to the declaration the antioxidants are the main active and beneficial components. Authors joining with the spirit of Saas Fee Declaration have worked out a complex examination system to study the antioxidant, free radical scavenging and inhibitory effects on lipid peroxidation of isolated components, natural plant extracts and herbal preparations. The members of the system are the followings: hydrogendonating ability, reducing power, chelating activity, free radical scavenging activities measured by chemiluminescence techniques, electron paramagnetic resonance / spin trapping studies, inhibition of lipid peroxidation induced in rat brain / liver microsomes, consequencies and treatment of experimental hyperlipidemia, morphological and histological studies. Only one measurement cannot give enough information about the antioxidant properties of the products. Complex in vitro examination of a novel antioxidant can draw pictures of correlation between the chemical structure and antioxidant effect, the mechanism of its favourable effect and its presumable role in prevention of lipid peroxidation. With the use of these results planning and execution of in vivo examinations should be more precise, practical and essential.