Conceptual process synthesis (CPS) is a very important issue in chemical process industries. During the last several decades, process synthesis has undergone major changes with respect to research issues as well as to the application domains. However, despite the publication of several excellent papers on process synthesis, such as Nishida et al. [AIChE J. 27 (3) (1981) 321], Westerberg [Comput. Chem. Eng. 13 (4-5) (1989) 365], and Johns [Chem. Eng. Progr. (2001) 59], there is a lack of an up-to-date overview paper devoted to the developments of conceptual process synthesis. This overview covers the main challenges and approaches in CPS that have emerged during the last two decades. This review is composed of five sections. The introduction outlines the history, milestones and development of CPS. It points out that the development of the conceptual design is carried out in the context of the three scales of process synthesis: the micro-, meso-, and macro-scale. The next section presents general design tasks. The methods, mainly knowledge-based, for solving a design problem are presented in the third section. In the fourth section, CPS is reviewed at the different scales through a discussion of the design tasks, achievements and most significant activities. The most interesting design topics are selected and discussed in more detail. Finally, this paper discusses the issues that will, possibly, dominate research into CPS in the near future, (C) 2003 Elsevier B.V. All rights reserved.