The surface engineering of polymers is still in development in order to modulate their properties to meet the requirements of the targeted applications. Lignocellulosic and agro-waste materials are increasingly studied as alternative to conventional ones employed for pollution remediation but also to develop original composites. In both cases, the surface of the material could be modified by coating or by covalent grafting. The diazonium chemistry has been successfully employed to modify the surface of carbon and metallic materials but its application to organic surfaces is still incipient. This chapter provides an overview of the recent developments in diazonium chemistry to modify polymer and biomass surfaces and its application in biosensor design, catalysis, and pollution removal.