Due to their excellent properties, polyurethane, cellulose and their composites have become widely used in previously unimagined areas. Given the fast-increasing consumption in recent years, the demand for advanced polyurethane- and cellulose-based composites is rapidly growing. Accordingly, this Special Issue seeks to showcase research papers, short communications and review articles focusing on innovations in polyurethane- and cellulose-based materials, including material synthesis, modification and engineering.