Natural bers are classied according to their source, which includes plants, animals, and minerals (Figure 7.2). Animal and mineral bers have extensive processing and material limitations constraining their effectiveness and use in a much similar manner to plant-based sources. Substantial research is being conducted on how to integrate bird feathers, for example, into biomatrices, which seem to have problematic processing issues due to varied ber proles that limit their overall usefulness. Furthermore, being brittle, mineral bers have fundamental processing limitations. In general, plant bers are the most attractive and abundant natural reinforcement in polymer composites. Many varieties of plant bers exist, and they can be divided into ve main categories depending on the part of the plant from which they are extracted [1-2], as shown in Figure 7.2.