Polymer blends are mixtures of at least two polymers and/or copolymers comprising more than 2 wt% of each macromolecular component. Most blends are immiscible, and need to be compatibilized. The compatibilization must not only ensure improvement in performance, but it must be reproducible, insensitive to forming stresses and repeated processing. This review on compatibilization of polymer blends is prepared in three parts : (i) description of the interface/interphase; (ii) compatibilization by addition of a copolymer (with a special emphasis on the use of block copolymers); and (iii) reactive compatibilization.