Apolipoproteins are proteins that bind lipids (oil-soluble substances such as fats, cholesterol and fat soluble vitamins) to form lipoproteins. They transport lipids in blood, cerebrospinal fluid and lymph.

The lipid components of lipoproteins are insoluble in water. However, because of their detergent-like (amphipathic) properties, apolipoproteins and other amphipathic molecules (such as phospholipids) can surround the lipids, creating a lipoprotein particle that is itself water-soluble, and can thus be carried through body fluids (i.e., blood, lymph).

In addition to stabilizing lipoprotein structure and solubilizing the lipid component, apolipoproteins interact with lipoprotein receptors and lipid transport proteins, thereby participating in lipoprotein uptake and clearance. They also serve as enzyme cofactors for specific enzymes involved in the metabolism of lipoproteins.

Apolipoproteins are also exploited by hepatitis C virus (HCV) to enable virus entry, assembly, and transmission. They play a role in viral pathogenesis and viral evasion from neutralizing antibodies.

Different lipoproteins contain different classes of apolipoproteins, which influence their function.

Apolipoprotein A-I (apoA1) is the major structural protein component of high-density lipoproteins (HDL), although it is present in other lipoproteins in smaller amounts. Apolipoprotein A-IV (apoA4) is present in chylomicrons, very-low-density lipoproteins (VLDL), and HDL. It is thought to act primarily in reverse cholesterol transport and intestinal lipid absorption via chylomicron assembly and secretion. ApoA-IV synthesized in hypothalamus is suggested to be a satiating factor which regulates the food intake of the rodent.

Apolipoprotein B plays a particularly important role in lipoprotein transport being the primary organizing protein of many lipoproteins.

Apolipoprotein C-III (apoC3) plays an important role in lipid metabolism specific in regulating the metabolism of triglyceride-rich lipoproteins (TRLs).