There is an intense discussion in the scientific and even more so in the public community as well as regulatory agencies about the potential benefits or detrimental effects of plant-derived compounds that may affect the endocrine system, especially estrogen signaling pathways. These so-called phytoestrogens are found in the normal western diet and predominantly in an eastern or soy-based diet and the potency of the isolated compounds to interact with the known receptors for estrogen varies tremendously. The estrogen receptors, ER alpha and ER beta, mediate the effects of endogenous estrogens, i.e. regulation of reproductive function, tissue development, cell proliferation and differentiation. In this review, in vitro test systems available to date for the screening of estrogenic and antiestrogenic activity including mechanism-based assays are described. The potency of phytoestrogens determined using these in vitro assays are compared with the potency of endogenous estrogens and results obtained in vitro are compared with effects in vivo. Finally, the impact of in vitro assays to determine estrogenicity on human hazard assessment is discussed as well as other non ER-mediated mechanisms that may contribute to potential beneficial or adverse effects of phytoestrogens in man.