Because of it's expressed immunommodulatory effect Echinacea is one of the most investigated medicinal plants. Despite numerous studies of it's chemistry, mechanism of action of main constituents, there is little evidence from clinical studies related to it's efficiency and safety. The main goal of this diploma thesis was to investigate the mechanism of immunomodulatory effect of Echinaceae species. Additionally, aims were to: explain metabolism, describe side effects and interactions which can emerge during usage of preparations which contain Echinacea and to show importance of quality control of herbal medicine used in clinical practice. The mechanism of immunomodulatory effect is based on increased phagocytosis, increased mobility of the T-lymphocytes and increased respiratory activity. Anti-inflammatory effect is based on up-regulation of anti-inflammatory cytokines and down-regulation of pro-inflammatory cytokines. Metabolism of main active components involves reactions catalyzed by cytocrome P450. Because of it's inhibitory effect on CYP3A4, interactions with drugs which are substrates of this enzyme are possible. Therapeutic potential of this interesting plant is based on results from numerous preclinical and growing number of clinial trials which are going in favor oEchinacae immunomodulatory effect. To maximize therapeutic potential of Echinacea and to minimize side effects and interactions we need better quality control of available preparations considering quantity, quality and type of constituents.