Chylomicrons are the ‘orphans’ of the lipoprotein family. Difficulty of measurement has impeded understanding of their metabolism. Plasma concentrations of chylomicrons and chylomicron remnants give no insight into the magnitude of substrate flux through these pathways. A defect in clearance of chylomicron remnants is probably an indication of a more generalized defect in lipoprotein metabolism. Accumulating evidence supports a relationship between abnormalities in the clearance from plasma of chylomicron remnants and accelerated progression of atherosclerosis. Methods using stable isotopes in appropriately formulated emulsions are providing valuable new information.