The chapter explores bio-composites, materials composed of natural-based reinforcements and resins, with at least one renewable component. It classifies bio-composites into three types: those with synthetic matrices and natural reinforcement, natural matrices and synthetic reinforcement, and entirely natural “green bio-composites.” The chapter emphasizes the importance of polymer matrix selection based on thermal properties, as natural fibers degrade above 200 °C. It details the structure, composition, and treatment methods of natural fibers to improve adhesion and reduce moisture sensitivity. A significant focus is placed on applying natural fiber-reinforced polymers (NFRPs) in the automotive industry, noting their benefits in reducing weight and emissions and enhancing sustainability. The chapter also describes processing techniques for thermoplastic and thermoset matrices, highlighting the structure, properties, and production of wood-plastic composites (WPCs).