Chapter in “Targeting of Drugs 6 Strategies for Stealth Therapeutic Systems: Proceedings of a NATO ASI Held in Cape Sounion Beach, Greece, June 24-July 5, 1997”. This book introduces strategies for stealth therapeutic systems. It presents a variety of approaches to evade the reticuloendothelial system and thus extend the circulation time of a variety of delivery systems including polymers, biopolymers, liposomes, and other nanoparticles or microparticles. The purpose is to provide the researchers in relevant fields with the basic concepts and advanced approaches of drug delivery systems, especially the protection of these delivery systems from the Mononuclear Phagocyte System (MPS). It is written for professional people who are working in drug delivery, gene therapy, and other related areas. The book contains a lot of updated information and summarizes it logically. It provides us with many exciting approaches of drug delivery systems in both therapeutic and diagnostic clinical work. It is very useful for researchers who are working on how to improve drug delivery systems in their studies.