Planning Support Systems: Retrospect and Prospect It has been nearly twenty years since the term planning support systems (PSS) first appeared in an article by Britton Harris (Harris 1989) and more than ten years since the concept was more broadly introduced in the academic literature (Harris and Batty 1993; Batty 1995; Klosterman 1997). As a result, the publication of a new book on PSS provides an excellent opportunity to assess past progress in the field and speculate on future developments. PSS have clearly become very popular in the academic world. This is the fourth edited book devoted...