Int. J. Business Continuity and Risk Management, Vol. 1, No. 2, 2010 113 Copyright © 2010 Inderscience Enterprises Ltd. On the boundaries of probabilistic risk assessment in the face of uncertainties, a case of piracy and armed robberies against ships in the Gulf of Aden Terje Aven* and Riana Steen University of Stavanger, 4036 Stavanger, Norway Fax: +47 518 31750 E-mail: terje.aven@uis.no E-mail: riana.steen@uis.no *Corresponding author Abstract: It is a common conception among many researchers and analysts, in particular social scientists, that using probabilistic risk assessment (PRA) when there are large uncertainties about likelihoods and outcomes is irrational, unscientific and potentially misleading. In this paper, we look more closely into this issue: the boundaries of PRAs when there are large uncertainties about the phenomena and processes analysed. Using an example of maritime piracy, we demonstrate that PRA also has an important role to play in risk management when the uncertainties are large. A paradigm shift is, however, required for how to conduct and use the risk assessments. The ambition of precise risk estimation has to be replaced by uncertainty assessments and characterisation. Keywords: probabilistic risk assessments; uncertainties; chances; piracy. Reference to this paper should be made as follows: Aven, T. and Steen, R. (2010) ‘On the boundaries of probabilistic risk assessment in the face of uncertainties, a case of piracy and armed robberies against ships in the Gulf of Aden’, Int. J. Business Continuity and Risk Management, Vol. 1, No. 2, pp.113–124. Biographical notes: Terje Aven is a Professor of Risk Analysis and Risk Management at the University of Stavanger, Norway. He has been an Adjunct Professor in Reliability and Risk Analysis at the universities in Trondheim and Oslo. He has many years of experience from industry, DNV and RF/IRIS. He has published a large number of papers (more than 100) in international peer- reviewed journals on risk, safety and reliability, and is the author of several books in the field. He received his Master’s degree (Cand. Real) and PhD (Dr Philos) from the University of Oslo in 1980 and 1984, respectively. Riana Steen is a PhD Student in Risk Management and Societal Safety at University of Stavanger, Norway. 1 Introduction Probabilistic risk assessment (PRA) is a formalised quantitative decision-support tool which is based on the estimation of probabilities and expected values for attributes like