Caveolae are flask-shaped invaginations present in the plasma membrane of many cell types. They have long been implicated in endocytosis, transcytosis, and cell signaling. Recent work has confirmed that caveolae are directly involved in the internalization of membrane components (glycosphingolipids and glycosylphosphatidylinositol-anchored proteins), extracellular ligands (folic acid, albumin, autocrine motility factor), bacterial toxins (cholera toxin, tetanus toxin), and several nonenveloped viruses (Simian virus 40, Polyoma virus). Unlike clathrin-mediated endocytosis, internalization through caveolae is a triggered event that involves complex signaling. The mechanism of internalization and the subsequent intracellular pathways that the internalized substances take are starting to emerge.