Plants abound in essential phytochemicals produced for their various vital functions. The same compounds seem also to be crucial for human health and disease. Recent human epidemiological and laboratory animal and cell studies on cancer and heart disease have highlighted the phytoestrogens--naturally occurring principles that share with steroidal oestrogens an ability to activate oestrogen receptors. The best known non-steroidal phytoestrogens include the isoflavones daidzein, genistein, formononetin and biochanin A, the coumestan coumestrol, and the lignans secoisolariciresinol and matairesinol. Acknowledging the potentially chemoprotective role of these non-nutrients, we have quantified all biologically important isoflavonoids and lignans in cereals, oilseeds and nuts, legumes, vegetables, fruits, berries and beverages such as tea, coffee and wine. In this chapter, we present a review of our studies on staple plant foods, indicating that plants contain, besides a wide range of chemicals with a number of biological properties, biologically active phytoestrogens--precursors of hormone-like compounds found in mammalian systems.