Remnant lipoproteins are reported to play a causative role in atherogenesis. They consist of chylomicron remnants and very low-density lipoprotein remnants. They are heterogeneous in composition and metabolic pathway and are rapidly taken up from the circulation as soon as they are formed in normal lipid metabolism. These characteristics make it difficult to separate and subsequently analyze remnant lipoproteins. Recently, however, we developed a simple and reliable immunoseparation method to separate remnant lipoproteins, and the cellular mechanism(s) by which they contribute to atherogenesis have been elucidated.