Polymer nanocomposites represent a transformative advancement in materials science, integrating nanoscale fillers into polymer matrices to achieve superior mechanical, thermal, and functional properties. These materials have gained significant attention for their applications in aerospace, automotive, biomedical, and renewable energy sectors. The classification of polymer nanocomposites is primarily based on the dimensionality of the fillers, including two-dimensional (2D), one-dimensional (1D), and three-dimensional (3D) nanostructures. Various fabrication techniques, such as melt blending, in situ polymerization, and solution processing, have been developed to optimize their dispersion and performance. A key focus of this chapter is the application of polymer nanocomposites in high-performance materials and emerging technologies such as solar cells, where they contribute to enhanced charge mobility, mechanical flexibility, and power conversion efficiency. Despite their advantages, challenges related to nanoparticle dispersion, interfacial bonding, and large-scale production remain critical barriers to widespread adoption. Addressing these challenges requires advancements in processing techniques, functionalization strategies, and environmental considerations.