The role of dietary fats and oils in human nutrition is currently one of the key issues related to diet and health. Nutritional fats and oils contain both saturated and unsaturated fatty acids, mostly of cis-configuration. Physiological functions of trans fatty acids in foods, especially their possible role in atherosclerosis, the level of blood cholesterol, and coronary heart disease is of concern, but still subject to controversy. Furthermore, the cancer prevention properties of conjugated linoleic acid isomers, present in small quantities in typical diets, remain to be confirmed. An overview on the occurrence and physiological considerations of trans fatty acids is given.