Natural fiber composites are a class of biocomposites consisting of bio-fibers reinforced with a synthetic matrix or a bio-based matrix. They offer a significant potential market in many fields, e.g. the construction industry, automotive industry, etc. Bio-fibers can be very short wood fibers from both deciduous and coniferous sources, straws from corn, from wheat and from rice crops, etc. From a commercial standpoint, the most viable structural fibers come from purpose-grown textile plants and some fruit trees. Most of the composite is heterogeneous, so that a coupling agent is necessary to enhance the interfacial interaction between the natural fiber and the polymer matrix. With the latest developments in technology, the concept of bio-based coupling agents becomes more important for a friendly environment. The first section of the chapter presents issues of a general point of view for natural fiber-reinforced composites in many systems. The second section is concerned with the theory of interfacial interactions between natural fibers and a polymer matrix. Then, finally, the third section explains why we use coupling agents in conventional examples and especially recent studies in bio-based examples.