Polymer nanocomposites is an emerging technological field offering high-performance materials with unique and innovative property combinations, ideal for numerous advanced applications. These include among others high-efficiency automotive and aerospace applications, food packaging, biomedical applications, drug delivery, biotechnology, wastewater treatment, environmental protection, advanced energy storage systems and electronic devices, gas/liquid barriers, fuel tanks, sensors, flammability reduction, chemical resistance, UV protective coatings, sports equipment, consumer goods and so on. The final properties as well as the suitable application of nanocomposites are directly depended on the used polymer matrix, the size and shape of the nanofillers, their used amount, their dispersion into the polymer matrix and the interfacial interactions. The highest effectiveness is usually achieved when nanoscale fillers are added in small amounts ranging from 0.5 up to 5 wt%. The individual properties of the nanofillers are also a crucial factor controlling the performance of the produced nanocomposites (improved mechanical strength, toughness, thermal stability, thermal conductivity, electrical properties etc.). This special issue will welcome all kinds of research works focusing on nanocomposites applications: theory and experimental findings, property studies, along with future development. Emphasis will be given on the processing-structure-property relationships, as well as on recent trends towards the enhancement of their final properties. Review articles by experts in the field are also warmly welcome. Keywords: nanocomposites; nanoparticles; functional nanoparticles; surface properties; interfacial interactions; nanoparticles dispersion; hybrid nanocomposites; mechanical properties; thermal properties; thermal stability; thermal conductivity; electrical properties; gas barrier properties.