Various types of loop reactors are presented, which can be operated in single-, two-, or multiphase modes. The fluid dynamic laws for the design of the reactors are communicated. The focus is on the jet loop, which is frequently used in reaction technology. Of central importance, for example, for the residence time distribution or the generation of interfaces in gas/liquid systems, is the loop circulation speed, which is particularly emphasized. Considerations are also made for scale-up. The importance of loop reactors for the chemical industry is demonstrated using examples.