An unsaturated fat is a fat or fatty acid in which there are one or more double bonds between carbon atoms of the fatty acid chain. Such fat molecules are monounsaturated if each contains one double bond, and polyunsaturated if each contains more than one.

Hydrogenation converts unsaturated fats to saturated fats, while dehydrogenation accomplishes the reverse. Unsaturated fats tend to melt at lower temperatures than saturated fats, which tend to be solid at room temperature.

Both kinds of unsaturated fat can replace saturated fat in the diet. Substituting unsaturated fats for saturated fats helps to lower levels of total cholesterol and LDL cholesterol in the blood.