This review focuses on polymer- and liposome-based nanoparticles used in targeted delivery of bioactive molecules, from drugs to siRNA to pDNA. The perspective centers around commercial and clinical successes, and a rationalization of these successes. Microparticulate systems are not covered, and only those applications that truly utilize the advantages of nano size are covered. "Stealth" systems dominate in this review, as most of the clinical successes are for passive targeting rather than for active targeting of tissue. The relevance of nano size to gene delivery is also discussed with relevant examples.