Ultraviolet-visible (UV-Vis) spectroscopy is a non-destructive and critical analytical tool for characterizing polymer matrix nanocomposites (PMCs), providing fundamental insights into their optical properties and nanofiller behavior. UV-Vis spectroscopy is a fundamental tool for characterizing the optical properties of polymer nanocomposites containing carbon-based nanomaterials (such as graphene or CNTs), nanooxides, and semiconductor nanocrystals. This method enables quantitative evaluation of light absorption properties, allowing researchers to determine critical parameters such as the band gap energy for nanooxides and semiconductors, and to evaluate the effects of the dispersion state and concentration of carbon nanomaterials on transparency or absorption. The importance of this method lies in its ability to investigate the electronic interactions between the polymer matrix and embedded nanofillers (such as carbon nanomaterials, nanooxides or semiconductor nanocrystals) and allows the quantification of key parameters such as band gap energies, absorption thresholds and light shielding efficiency. In this paper, the optical properties of nanocomposites containing carbon-based nanomaterials, nanooxides and semiconductor nanocrystals as fillers will be investigated by UV-Vis spectroscopy with various examples.