This chapter presents elements from chemical reaction engineering necessary to consider in the conceptual design of a process as a system. The selection and the design of the reactor must be done in the context of interactions with the whole process, particularly with the separation system. The preliminary section reviews basic concepts with regard to stoichiometry, kinetics and chemical equilibrium. Then, design principles are discussed in the field of homogeneous reactions with reference to ideal reactor models, followed by heterogeneous reactions, where specific modelling problems are highlighted. Attention is paid to thermal design issues, related to safety and heat integration. Guidelines for the reactor selection are presented. The final section is devoted to new systematic methods developed by the process systems community.