The biopolymers chitin and chitosan, derived from crustacean shells, insects, and fungi, have gained significant attention as sustainable, high-performance fillers. Although chitin is abundant in nature, it is not soluble in water or organic solvents, limiting its direct application. It is possible, however, to convert chitin into chitosan, a water-soluble derivative, that is more versatile than chitin alone. Since chitin whiskers are both mechanically strong and biocompatible, they are used in various industries, including packaging, automotive, aerospace, and biomedical engineering. Throughout this chapter, chitin and chitosan whiskers are described in terms of their preparation as well as their characterization and applications in rubber, thermoplastic, and thermoset blends. Several key processing techniques including solution casting and melt mixing are discussed, along with the importance of interfacial adhesion and uniform dispersion. Among the many benefits of these biopolymers is their biodegradability and low carbon footprint, which align with the growing demand for sustainable materials. Among the applications are improving mechanical properties in automotive components, developing eco-friendly packaging, and developing advanced biomedical devices. Finally, the chapter illustrates how chitin and chitosan whiskers offer performance and sustainability benefits in a multifunctional, renewable material.