This review highlights the properties of nanoparticles used in targeted drug delivery, including delivery to cells as well as organelle targets, some of the known pharmacokinetic properties of nanoparticles, and their typical modifications to allow for therapeutic delivery. Nanoparticles exploit biological pathways to achieve payload delivery to cellular and intracellular targets, including transport past the blood-brain barrier. As illustrative examples of their utility, the evaluation of targeted nanoparticles in the treatment of cancers and diseases of the central nervous system, such as glioblastoma multiforme, neurovascular disorders, and neurodegenerative diseases, is discussed.