Most people think of soy when they hear the term phytoestrogens. This view is simplistic as there are several classes of phytoestrogens, each having fairly specific food sources, including those other than soy. The main classes of phytoestrogens are isoflavones which includes genistein and daidzein, the mammalian lignans enterolactone and enterodiol, and coumestans. There are other phytochemicals that may also have some estrogenic potential, but I will mainly be discussing isoflavones and lignans. Genistein, daidzein, enterolactone, and enterodiol are the metabolites of these plant precursors and can be measured in mammalian fluids. Genistein and daidzein are metabolized from their plant precursors biochanin A and formononetin and the lignans enterolactone and enterodiol from the plant precursors matairesinol and secoisolariciresinol. Isoflavones are present in plants in the bound glycoside form, but become active after removal of the sugar residue. Fermentation by intestinal bacteria renders them liable to absorption. They are measurable in serum, and excreted and measurable in the urine. Lignans exist as minor constituents of many plants and form the building blocks for lignins in the plant cell walls. Plant lignans are metabolized in the mammalian gut to form the mammalian lignans enterolactone and enterodiol. Metabolism and reabsorption of the mammalian lignans can be affected by anything that affects the gut, such as antibiotic use, or constipation.