Since the early 1990s, 'Technology Foresight' exercises with special emphasis on the use of Delphi surveys have played an important role in science and technology (S + T) policy across Europe in an effort to focus resource allocation. Yet, none of the estimates made in the European Delphi surveys have been formally assessed in retrospect, while this process has been incorporated into the Japanese surveys since 1996. Taking the UK Technology Foresight Programme, this research sets out to assess the estimates of three of the fifteen panel Delphi surveys. Whilst on average 2/3 of Delphi statements were predicted to be realised by 2004, it will be shown that only a fraction of these statements had been realised by 2006. Based on the evidence collected from the published panel reports, the 'Hindsight on Foresight' survey conducted by OST in 1995 and interviews with panel members, it will be argued that the overwhelming majority of estimates were overly optimistic. While optimism and strategic gaming of experts is the most convincing explanation for these results, process factors were also explored, including the quality of expert panels used, the Delphi statements and the respondents of the Delphi questionnaire. It is argued that at least the issue of short-range optimism and strategic gaming of experts should be addressed in future Delphi exercises, as decision makers relying on expert advice cannot deal with this issue alone. © 2009 Elsevier Inc. All rights reserved.