Dietary fats are chemical compounds composed of carbon, hydrogen, and oxygen, and are found in a variety of foods, including animal products, seeds, nuts, and vegetables. The amount and type of dietary fat that should be present in a healthful diet has been the source of tremendous controversy in recent years.

Dietary fat supplies 9 calories per gram (compared to 4 calories per gram from carbohydrates and protein) and aids in the absorption and transport of the fat-soluble vitamins (vitamin A, vitamin D, vitamin E, and vitamin K) and other fat-soluble nutrients. Fat improves the taste and feel of food and provides a sense of satiety; dietary fat also provides essential fatty acids that the body cannot synthesise on its own, but requires for many normal physiological processes. Eating too much of the wrong kinds of fat increases the risk of obesity, heart disease, and cancer. As a result, nutritionists and physicians often recommend that people reduce their total fat, saturated fat, and trans fat intake.

Dietary fats are divided into four main categories: saturated fats, monounsaturated fats (MUFAs), polyunsaturated fats (PUFAs), and trans (or hydrogenated) fats. These fats are distinguished by the structure of the fatty acids they contain. Fatty acids are made up of carbon atoms arranged in chains of varying length, to which a variable number of hydrogen and oxygen atoms are joined. The structure of the carbon chain may or may not include a double bond between the carbon atoms; a chain containing double bonds will hold fewer hydrogen atoms than a chain containing only single bonds. Foods often contain a mixture of different fatty acids, but can be categorized by the fatty acids most prevalent in the food.