Polymers fall into three broad categories: natural, semi-synthetic and synthetic. Polymers are widely used in pharmaceutical systems as drug carriers, adjuvants, suspending and emulsifying agents. The word "polymer" which means "many parts", is derived from the Greek words poly, meaning "many," and meros, meaning "parts". Polymers are widely found in nature and occur in many forms. The human body contains many natural polymers such as proteins and nucleic acids, while cellulose is the main structural component of plants. Cellulose, starch, lignin, chitin, and various polysaccharides are of natural origin. These materials and their derivatives offer a wide range of properties and applications in drug delivery. Natural polymers are usually biocompatible and biodegradable. In this chapter, sources and applications of natural biopolymers in particulate drug delivery systems such as microparticles and nanoparticles, which are currently widely investigated as drug delivery systems, were discussed.