A brief review of reactive processing of polyolefins, which are the most widely used polymers today, is given in this paper from the role of flow field in processing and the characterization of the modified chain topologies. The materials are mainly focused on polyethylene (PE), polypropylene (PP) and polyolefin elastomer (POE) here. Aspects of reactions, coupling/crosslinking and scission are discussed both in terms of the mechanism and kinetics under different flow conditions.