During the recent decades, polymers and polymer nanocomposites have gained a prominent position in advanced industries, including optoelectronics, due to their characteristics such as light weight, chemical stability, easy processability, and low production costs. This study adopts an analytical-review approach to investigate the birefringence phenomenon in polymers and polymer nanocomposites and its role in the design of advanced optical materials. Birefringence, which arises from structural anisotropy and the orientation of molecular chains or nanofillers, enables precise control over the behavior of light within the material. This article first details the theoretical foundations for birefringence, including the Lorentz-Lorenz equation for predicting the refractive index, the Abbe number as a measure of optical dispersion, and stress-birefringence relationships. Subsequently, the main mechanisms governing birefringence, including the Pockels electro-optic effect, stress- and flow-induced birefringence, and anisotropy caused by the arrangement of nanoparticles, are analyzed. Methods for identifying and measuring this phenomenon, from polarized optical microscopy to interferometry and spectroscopic ellipsometry, are also reviewed. In the applications section, the key role of birefringence in optical sensors, display technologies, stress analysis, and smart materials is examined.