Diblock copolymers can form nanoparticles, that is, micelles and nanospheres, that are being studied as carriers for hydrophobic drugs and genes. The synthetic carriers mimic the spherical, supramolecular core/shell structure of lipoproteins and viruses. Hence, diblock copolymer nanoparticles may be functional, having the ability to solubilize, protect, and release drugs at sustained rates. Several studies have illustrated prolonged residence times in blood for diblock copolymer nanoparticles. They have also enhanced drug effects in animals. Diblock copolymer nanoparticles are potentially useful carriers for site-specific drug delivery.