A wide variety of drug delivery systems have been developed, each with its own advantages and limitations, but the important goals of all of the systems are to enhance bioavailability, reduce drug toxicity, target to a particular organ, and increase the stability of the drug. The development of nanostructured drug carriers have grasped increased attention from scientific and commercial organizations due to their unique ability to deliver drugs and challenging molecules such as proteins and nucleic acids. These carriers present many technological advantages such as high carrier capacity, high chemical and biological stability, feasibility of incorporating both hydrophilic and hydrophobic substances, and their ability to be administered by a variety of routes (including oral, inhalational, and parenteral) to provide controlled/sustained drug release. Moreover, applications of nanoparticulate formulations in enhancing drug solubility, dissolution, bioavailability, safety, and stability have already been proven. In the view of their multifaceted applications, the present review aims to discuss and summarize some of the interesting findings and applications, methods of preparation, and characterization of various nanostructured carriers useful in drug delivery. Included in this discussion are polymeric nanoparticles, solid lipid nanoparticles, nanostructured lipid carriers, dendrimers, cyclodextrins, fullerenes, gold and silica nanoparticles, and quantum dots. Because there are likely to be new applications for nanoparticles in drug delivery, they are expected to solve many problems associated with the delivery of drugs and biomolecules through different delivery routes.