One of the advantages of using natural fibers in composites is that they are biodegradable and renewable. Many applications for composites, however, require high performance and durability. While our attention is being drawn to renewable, biodegradable and sustainable resources for materials, we are faced with the concern about durability. For the most part, we have designed and used natural fiber-based composites accepting performance limitations such as dimensional instability, ultraviolet and biological degradation, and thermal instability. However, with a better understanding the relationship between chemistry, properties, and performance of natural fibers, we can produce a new generation of value added composites that will be performance driven and durable. Properties such as dimensional instability, flammability, biodegradability, and degradation caused by acids, bases, and ultraviolet radiation are all a result of chemical degradation reactions (hydrolysis, oxidation, dehydration, and reduction) which can be prevented or, at least slowed down if the cell wall chemistry is altered. This approach is based on the premise that the properties of any resource are a result of the chemistry of the components of that resource. In the case of natural fibers, cell wall polymers, extractives, and inorganics are the components that, if modified, would change the properties of the resource. Based on performance requirements, chemical modifications can be carried out to change the chemistry of the resource that will result in a change in performance. Many chemicals have been used to chemically modify natural fibers to improve performance including anhydrides, acid chlorides, ketene carboxylic acids, isocyanates, aldehydes, beta-propiolactone, acrylonitrile, and epoxides [16-19,28]. After reaction, the natural fibers are made into a composite and the composite has greatly improved dimensional stability and biological resistance. Chemistries can also be selected to improve thermal resistance and resistance to UV radiation.