Lipids (from the Greek, meaning “fat”) comprise a heterogeneous group of organic compounds, insoluble (or sparingly soluble) in water, but soluble in nonpolar solvents such as chloroform, ether, and benzene. Lipids differ in their structure, but all have pronounced nonpolar groups, or both nonpolar and polar groups. Thus, lipids are either hydrophobic or amphipathic compounds. Unlike proteins and polysaccharides, lipids are small molecules, but they have a strong tendency to associate through noncovalent interactions. As major parts of their structure, many lipids have long hydrocarbon chains that may be saturated or unsaturated. Some lipids have specific roles and high biological activity. Among these, we find several vitamins and hormones. In addition, lipids have a number of general biological functions.