The interaction and adhesion between the fiber and matrix has a significant effect in determining the mechanical and physical behavior of fiber composites. The effect of the interface and interphase depends on several factors such as chemical composition (functional groups), molecular structure characteristics (branching, molecular weight distribution, cross-linking), and details of its physical state (above or below T_g, nature and degree of crystallinity). Natural fibers have complex and varying chemical structures that have uneven surface topographies. This creates difficulties in using single fiber composite testing to accurately evaluate the interfacial shear strengths, except for comparisons. A review of our interphase related research in natural fiber composites is presented. When using coupling agents it is well known that the tensile and flexural strengths increase dramatically in natural fiber reinforced composites. However, in the case of modulus, the results are more complex. For two ethylene-propylene impact copolymers, the uncoupled systems had much higher Young's moduli than the coupled systems. The dynamic storage moduli of the uncoupled impact polymers were higher than the coupled composites at temperatures up to about 50°C. At higher temperatures the presence of the coupling agent resulted in higher storage moduli. Transcrystallinity may play an important role in this phenomenon. Creep and other long-term properties are also affected by the quality of the interphase, although the level of improvement decreases with an increase in the molecular weight of the matrix polymer. Coupling agents reduced the rate of water absorption and the moduli were less affected in blends with a higher concentration of coupling agents.