Complex systems require more time and resources to develop and are more likely to go wrong and the resulting faults are more difficult to correct. Complexity is a property that we all intuitively recognise, but yet it is ill-defined and this makes it difficult to obtain the measures of complexity that are needed to support management decisions regarding the allocation of resources to projects such as the development of chemical processes. In this paper, we differentiate between complicatedness and complexity, and between process and plant. Using words and examples from a variety of contexts (including mechanical engineering and management science), we try to illustrate the properties of each, such that in later papers we can provide unambiguous measures of system complexity to permit the comparison of process options. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B,V All rights reserved.