Lipids are organic compounds of biological nature that includes fats, oils and waxes. They are insoluble in water but soluble in nonpolar solvents such as ether, chloroform and benzene. Lipids are utilizable by living organisms. This chapter covers the biological functions of lipids, fatty acids, essential fatty acids, biosynthesis of fatty acids, lipids classification, properties of lipids, lipid metabolism, disorders of lipid metabolism, ketone bodies, cholesterol, atherosclerosis, lipoproteins, and phospholipids. The fatty acids are the basic units of lipid molecules. Fatty acids are derivatives of aliphatic hydrocarbon chain that contains a carboxylic acid group. Fatty acids required in the diet are called essential fatty acids (EFA). A lipoprotein is a biochemical assembly that contains both proteins and lipids. The lipids or their derivatives may be covalently or non-covalently bound to the proteins. Many enzymes, transporters, structural proteins, antigens, and toxins are lipoproteins.