The process industry faces new challenges. In particular, environmental objectives call for processes with much higher conversion and better selectivities to minimize the release or removal of products to a degree which would not be economic without considering the environmental objectives. These challenges call for a better understanding of the design basis. The paper analyzes by examples the strength of the "state-of-art" of chemical reactor engineering which as examples formed the basis for progress in ammonia synthesis and in steam reforming. The importance of an integrated approach in process development is illustrated by examples from synthetic gasoline and direct and indirect manufacture of methanol. The new challenges to reactor engineering require more research in a field which has been developed almost to maturity. The better understanding of the micro-kinetics of catalysis has also revealed a complexity which is difficult to describe by the conventional approach. This is in particular true for "ppm reactions" in which one component should be removed down to a ppm level. There is a need to integrate the conventional reactor design models with models describing the fluid mechanics, in particular for reactions carried out with critical mixing (SCR, hydrotreating). Breakthroughs may come from new concepts in combining separation with catalysis and use of transient reactors. This is illustrated by a new solid state alkylation process. In conclusion, chemical engineering science needs more explorative research, if it should not become mature.