A nanomedical device should perform multiple functions, so a multifunctional nanoparticle system can be constructed from the inner core outward. The core can contain a drug, while outer layers provide targeting ligands, such as antibodies, peptides, or aptamers directed against receptors overexpressed on diseased cells. Aptamers have advantages over antibodies like uniform activity between batches. Peptides can also serve as targeting ligands if they recognize receptors. Other ligands like folate or transferrin can bind their cognate receptors, but compete with circulating levels; antibodies are more specific but expensive. A variety of conjugation strategies can be used to attach targeting ligands to nanoparticles.