Abstract :The past half of a century has seen increasing attention focused on the development of polymer nanoparticle based drug delivery systems as being capable of improving the efficacy and obviating the systematic side effects of a wide range of antineoplastic drugs in human cancer therapy. This has led to the emergence of multifunctional “smart” nanoparticles for carrying out protective polymer capping, targeting ligand, diagnostic label, stimuli-sensitive segments, image contrast agent, and other demanding functional groups. In this article we introduce the basic knowledge and general features of polymer nanoparticles for the purpose of drug delivery. In addition, various types of polymer nanoparticles used in drug delivery systems are presented. The criteria for the development of the required polymer nanoparticles is also summarized.