Many exothermic reactions-halogenations, oxidations, polymerizations, hydrogenations-are carried out at very large scale. This paper considers features of the design and operation of such large reactors. Several questions arise when exothermic reactions are carried out at the high gas rates typical of many industrial processes. The difficulties include the prediction of power input and its effectiveness in achieving the desired fluid circulation, the control of dispersion and coalescence, the proper understanding of void distribution and interphase mass transfer, appropriately dealing with any solid products and factors such as micro and mesomixing that can control the purity of the reaction product. This paper examines physical and chemical aspects of the processes involved. These problems are reviewed in the context of relevant research findings and highlights some unresolved questions.