I. INTRODUCTION Fatty acids in foods occur mainly in the form of esters with the alcohol glycerol. The predominant esters are triacylglycerols accompanied by minor amounts of mono-and diacylglycerols and free fatty acids. In fact, most fats and oils can be classi ed as mixtures of mixed glycerides. To understand the chemical and physical properties of these food fats and oils, we have to go back to the properties of the component fatty acids. However, many of the properties of food fats and oils are not simply a re! ection of the properties of the component fatty acids but are also in! uenced by the nature of their incorporation into the acylglycerols. In this chapter, therefore, both the properties of fatty acids and their behavior in the form of acylglycerols will be covered.