The fluorescent, magnetic, and conductive properties of nanoparticles can transform polymer-based materials into composites with higher levels of sophistication than found in polymers alone. The ligand chemistry of nanoparticles is critically important in the development of polymer-nanoparticle composites to prevent nanoparticle aggregation and direct their assembly within polymers. Nanoparticle ligands can also prevent aggregation in solution and direct the assembly of nanoparticles at fluid-fluid interfaces, where interfacial chemistries can be performed to provide new routes to ultrathin composite sheets and capsules. (c) 2006 Wiley Periodicals, Inc.