Polymeric nanoparticle technology has evolved from drug carrier design to advanced multifunctional macromolecular structures. They enable drug delivery and release of a bioactive under spatio-temporal control rather than just passive release by a long-circulating carrier. As such, the carrier is enabling the biomolecule or the bioactive to carry out its designed biological function. Due to their small size nanoparticles may also induce perturbations of biological systems different from any other biomaterials, therefore opening up new biomedical applications as well as raising concerns about adverse effects.