Polymer nanocomposites and nanostructured polymers represent a transformative frontier in materials science, merging polymers with nanoscale fillers to enhance mechanical, thermal, and barrier properties. This chapter delves into the synthesis, characterization, and applications of these advanced materials, highlighting their unique structural attributes and multifunctional capabilities. By examining various nano fillers such as carbon nanotubes, graphene, and nanoparticles, the chapter elucidates their role in improving polymer matrix properties. Key topics include fabrication techniques, interfacial interactions, and the impact of nano structuring on material performance. The chapter also explores the burgeoning applications of polymer nanocomposites in fields such as aerospace, automotive, electronics, and biomedical engineering, emphasizing their potential to revolutionize product design and functionality. The chapter aims to provide a foundational understanding of the principles and progress in the field of polymer nanocomposites and nanostructured polymers.