High Density Lipoproteins, or HDL, are commonly thought of as "good cholesterol". These particles are composed of lipids and proteins (lipo-proteins) and are the smallest of their class. There role is to transport fat molecules from cells into the liver, where they can be excreted from the body. In the context of health, a high concentration of HDL is associated with a decrease in atherosclerosis, thereby decreasing the risk of vascular diseases.