Trimethylamine N-oxide (TMAO) is a small organic compound whose concentration in blood increases after ingesting dietary l-carnitine and phosphatidylcholine. Recent clinical studies show a positive correlation between elevated plasma levels of TMAO and an increased risk for major adverse cardiovascular events defined as death, myocardial infarction, or stroke. Several experimental studies suggest a possible contribution of TMAO to the etiology of cardiovascular diseases by affecting lipid and hormonal homeostasis. On the other hand, TMAO-rich seafood, which is an important source of protein and vitamins in the Mediterranean diet, has been considered beneficial for the circulatory system. Although in humans TMAO is known mainly as a waste product of choline metabolism, a number of studies suggest an involvement of TMAO in important biological functions in numerous organisms, ranging from bacteria to mammals. For example, cells use TMAO to maintain cell volume under conditions of osmotic and hydrostatic pressure stresses. In this article, we reviewed well-established chemical and biological properties of TMAO and dietary sources of TMAO, as well as looked at the studies suggesting possible involvement of TMAO in the etiology of cardiovascular and other diseases, such as kidney failure, diabetes, and cancer.