Of any plant, soy contains the largest concentration of isoflavones, a class of phytoestrogens. Phytoestrogens are structurally similar to estradiol and mimic its effects. Soy and phytoestrogens receive increasing attention due to the health benefits associated with their consumption. Here we review the data collected on the effects of soy and phytoestrogens on glucose and lipid metabolism and their possible mechanisms of action. Overall, there is a suggestive body of evidence that soy and dietary phytoestrogens favorably alter glycemic control, improve weight and fat loss, lower triglycerides, low density lipoprotein (LDL) cholesterol and total cholesterol. However, these results must be interpreted with care, and additional evidence is needed before a firm conclusion can be drawn. In particular, since not all activities related to soy can be assigned to the estrogenic-like activity, further studies are needed to identify firstly which soy constituent(s) improve metabolic parameters when ingested and secondly, which are the mechanisms whereby dietary soy improves metabolic-related conditions like obesity and diabetes. Finally, the potential detrimental effects of soy and phytoestrogens are briefly discussed.