The lipoprotein transport system is vital to the delivery of the hydrophobic fats that are carried in the aqueous environment of the blood. The lipoprotein particles that comprise this system are polydisperse and contain triglycerides, free and esterified cholesterol, phospholipids and proteins. Inborn errors in the lipoprotein transport system lead to alterations in both the steady state concentrations of the various lipoproteins and in the metabolism of these particles. These inborn errors lead to both hyperlipoproteinemia and hypolipoproteinemia. Profound changes in the ambient lipoprotein concentrations have a variety of clinical manifestations. The present study protocol is designed to permit a full evaluation of the lipids, lipoproteins, and apolipoproteins, in patients with potential genetic defects in these processes. The protocol will also permit training of students, staff clinicians, physician assistants, nurse practitioners, dieticians and post-doctoral fellows in the evaluation and treatment of patients with dyslipidemias. The study population will include patients which are referred to the Lipid Service from private care providers, academic institutions, or the NHLBI Lipid website, with any of the following potential lipid abnormalities or clinical stigmata associated with dyslipoproteinemias: a) increased plasma levels of cholesterol, triglycerides, HDL-cholesterol or LDL-cholesterol b) decreased plasma concentrations of cholesterol and HDL-cholesterol c) postprandial hyperlipidemia or d) eruptive xanthomas, xanthelasma, tuberous or tendinous xanthomas, or corneal opacities.