An unsaturated fat is a fat or fatty acid in which there are one or more double bonds in the fatty acid chain. A fat molecule is monounsaturated if it contains one double bond, and polyunsaturated if it contains more than one double bond.

Where double bonds are formed, hydrogen atoms are eliminated. Thus, a saturated fat is "saturated" with hydrogen atoms. In cellular metabolism hydrogen-carbon bonds are broken down - or oxidized - to produce energy, thus an unsaturated fat molecule contains somewhat less energy than a comparable sized saturated fat.

The greater the degree of unsaturation in a fatty acid (ie, the more double bonds in the fatty acid), the more vulnerable it is to lipid peroxidation (rancidity). Antioxidants can protect unsaturated fat from lipid peroxidation. Unsaturated fats also have a more enlarged shape than saturated fats.

Double bonds may be in either a cis or trans isomer, depending on the geometry of the double bond. In the cis conformation hydrogens are on the same side of the double bond, whereas in the trans conformation they are on opposite sides. Saturated fats are popular with manufacturers of processed foods because they are less vulnerable to rancidity and are generally more solid at room temperature than unsaturated fats. Unsaturated chains have a lower melting point, hence increasing fluidity of the cell membranes.

Both monounsaturated and polyunsaturated fats can replace saturated fat in the diet, though trans unsaturated fats should be avoided. Replacing saturated fats with unsaturated fats helps to lower levels of total cholesterol and LDL cholesterol in the blood. Trans unsaturated fats are particularly bad because the double bond stereochemistry allows the fat molecules to assume a linear conformation which leads to efficient packing (i.e., plaque formation). The geometry of the cis double bond introduces a bend in the molecule precluding stable formations. Natural sources of fatty acids are rich in the cis isomer (olive oil).

Saturated fats (like butter or lard) and fatty acids with trans double-bonds (like margarine) tend to be solids at room temperature, whereas natural fatty acids with cis double-bonds (like vegetable oils) tend to be liquids. By artificially hydrogenating vegetable oils, the food processing industry reduces the number of double bonds and causes the formation of trans fatty acids. Hydrogenation results in margarines that are more solid and less vulnerable to rancidity. Hydrogenation results in peanut butter with a trans-fat-containing oil that does not separate from the peanut paste. But when trans fatty acids are incorporated into cell membranes, the membrane fluidity is reduced and the cells do not function as well. Not all trans fatty acids in the diet are due to food processing. For example, natural butter is 5% trans fat.

Examples of unsaturated fats are palmitoleic acid, oleic acid, linoleic acid, and arachidonic acid. Foods containing unsaturated fats include avocado, nuts, and vegetable oils such as soybean, canola, and olive oils. Meat products contain both saturated and unsaturated fats.

Although unsaturated fats are healthier than saturated fats, the old Food and Drug Administration (FDA) recommendation stated that the amount of unsaturated fat consumed should not exceed 30% of one's daily caloric intake (or 67 grams given a 2000 calorie diet). The new dietary guidelines have eliminated this recommendation.

Most food contain both unsaturated and saturated fats. Marketers only advertise one or the other, depending on which makes up the majority. Thus, various unsaturated fat vegetable oils, such as olive oils, also contain saturated fat.

Insulin resistance correlates positively with monounsaturated fat (especially oleic acid) and negatively with polyunsaturated fat (especially arachidonic acid) in the phospholipids of human skeletal muscle.