Polymer nanocomposites can be defined as materials in which nanoscopic inorganic particles, in at least one dimension, are dispersed in an organic polymer matrix to improve its performance properties. Polymer nanocomposites represent a new alternative to conventional filled polymers. Because of their size, the filler dispersion that nanocomposites exhibit markedly improves their properties when compared to pure polymers [1–7] or their traditional composites. These properties include increased tensile strength, conductivity and thermal stability as well as decreased flammability. This special issue focuses on the synthesis, characterization, electrical properties, and applications of polymer nanocomposites such as sun protection, resistive switching device, wastewater treatment, and biosensors. For the topics of synthesis and characterization of polymer nanocomposites, hybrid composites can be used as effective protective materials against ultraviolet rays since they have a low transmission and high reflection. Also, the result proves that hybrid composites, Nia/ZnO, are an excellent material for ultraviolet ray protection. For the topics of the structure and electrical properties of polymer nanocomposites, WORM/volatile static random access memory (SRAM) conversion was triggered by the composite of the oxadiazole small molecule. FTO/PMMA/Ag device possesses nonvolatile WORM memory behavior, while the FTO/PMMA+ oxadiazole/Ag device shows vastly different volatile SRAM feature. The FTO/PMMA/Ag and FTO/PMMA+ oxadiazole/Ag memory devices both exhibit high ON/OFF ratio nearly 104. The results manifest the significance of …