Isoflavones are diphenolic compounds found in most plants and in physiologically relevant concentrations in legumes. Their chemical structure enables them to bind weakly to mammalian estrogen receptors (ERs) with a preference for ERβ. Isoflavones have been shown to exert biological effects via agonistic and antagonistic genomic mechanism plus a variety of non-genomic mechanisms on the cardiovascular and skeletal systems, on menopausal vasomotor systems and on breast and prostate cancer. The true magnitude of these effects in vivo is complicated by variable absorption, equol-producer status and the nature of the parent isoflavone compounds ingested. Dietary and supplementary isoflavones have a low incidence of side effects.