The term “plastic” comprises usually a particular polymer and often different additives to create formulations and compositions to manufacture adjusted plastic products with the required properties for the intended field of application. As potential plastic applications are infinite, additive classes are numerous as well. Several additive classes hold a fundamental importance and are essential components of most plastics, others are only used as specific ingredients for selected polymers or for niche applications. The chapter provides an overview on plastic additives considering purpose and application fields, chemical structures, mode of action, individual advantages and risks, interaction of additives with the polymer and other components, usual loadings and concentrations. Environmental and circular economy aspects including developments and future trends are highlighted. Emphasis is placed on high-volume additives and large-scale applications, however elementary information is presented for special additives as well.