Introduction to chemical reactions on macromolecules. Changes in chemical structure of mers in the polymer chain. Branched and grafted copolymers. Crosslinked macromolecules - types of bonds and methods of crosslinking. Exchange and destruction reactions on polymer chains. Overview of reaction mechanisms and kinetics. Grafting monomers on polymers. Modifications by organic peroxides, silanes, reactive antioxidants etc. Polymer blends and alloys. Reactive extrusion. Relations among structural parameters, reactivity and resulting physical properties of modified polymers. Perspectives and developmental trends in the field.