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 to the topic following Brail and Klosterman (2001), Geertman and Stillwell (2003), and a third by Brail (2008). Papers devoted to PSS have been published in the leading planning journals and the topic has become a regular theme at academic conferences around the world; it has even spawned intellectual o- spring such as spatial planning and decision support systems (SPDSS) and public participation planning support systems (PP-PSS). However, as Geertman and Stillwell point out in their introductory chapter, the experience with PSS in the world of professional practice has been disappointing. A substantial number of PSS have been developed but most of them are academic p- totypes or 'one off' professional applications that have t been adopted elsewhere.