Natural polymers, especially polysaccharides, are currently attracting much attention for their fundamental properties for pharmaceutical applications due to their relatively low-cost, tunable functionality, biocompatibility, biodegradability, non-citotoxic effect. This book focuses on the state-of-art of natural polymers (cellulose and derivatives, hemicelluloses and chitosan) use as excipients in the formulation of drug delivery systems, mainly in nanoparticle drug delivery systems. Moreover, the state-of-art of these polymers (polysaccharides) in antitumor drugs delivery for cancer therapy has been reviewed and discussed.