The human apoCs (ie, apoC1, apoC2, and apoC3) are often portrayed as members of 1 consistent protein family because of their similar distributions among lipoprotein classes, their low molecular weights, and coincident purification. The human apoCs are protein constituents of chylomicrons, VLDL, and HDL. In comparison with the intensely studied apoE, apoB, and apoA1, which play important roles in the development of hyperlipidemia and atherosclerosis, only modest attention has been paid so far to the roles of the apoCs in lipoprotein metabolism. Many of the studies regarding the functional properties of apoCs have been hampered by methodological problems dealing with purification, quantification, and their poorly understood association with hyperlipidemia and other lipoprotein disorders. In the past few years, however, new insights into the metabolic properties of apoCs have been provided, in particular by the technologies of transgenesis and gene targeting in mice.