The scale-up of a chemical reactor takes place in the process of transferring a process engineering concept to a production plant that has previously been successfully tested on a laboratory or pilot scale. Thus, the scale-up of chemical reactors is an integral part of any process development. This chapter presents the methodology of scale-up for newly developed processes. It is shown that the reactor development diverges significantly in the practical approach depending on the desired production quantity. Therefore, in terms of methodology in reactor development, a distinction is made between the field of bulk chemistry, with examples of continuously operated fixed bed and fluidized bed reactors for heterogeneous gas phase reactions, and the field of fine chemistry with batch-operated stirred tank reactors for liquid phase reactions.