This chapter explores the types of nanofillers and matrices used in polymer nanocomposites, emphasizing their essential role in enhancing material properties for high-performance applications. Beginning with an introduction to polymer nanocomposites, it highlights the significance of nanoscale fillers in improving mechanical, thermal, electrical, and barrier characteristics. The first section categorizes nanofillers, examining their structural and functional features. Inorganic nanofillers—such as layered silicates, carbon-based materials (carbon nanotubes, graphene, and fullerenes), and metal or ceramic nanoparticles—offer reinforcement, conductivity, and stability. Organic nanofillers, including cellulose nanocrystals, dendrimers, and polyhedral oligomeric silsesquioxane (POSS), are noted for biodegradability and potential in sustainable composites. Hybrid nanofillers like core-shell nanoparticles and multicomponent systems provide multifunctionality. The chapter then investigates polymer matrices, discussing thermoplastics, thermosets, elastomers, and biodegradable polymers, with emphasis on compatibility with nanofillers for specific property enhancements. Key filler-matrix interactions are explored, focusing on surface functionalization to improve bonding and overall performance. A review of synthesis and processing techniques, including in situ polymerization, melt compounding, and solution blending, reveals their influence on filler dispersion and composite quality. Performance attributes such as strength, thermal stability, and conductivity are examined in relation to filler and matrix selection. Applications in automotive, aerospace, electronics, biomedicine, and sustainable packaging are highlighted, alongside challenges like dispersion, scalability, and environmental impact. The chapter concludes with future directions in sustainable nanofillers and biodegradable matrices, offering a foundation for advancing polymer nanocomposite design and applications across industries.