‘‘Penetrating so many secrets,we cease to believe in the unknowable.But there it sits nevertheless,calmly licking its chops.’’– H.L. Mencken, Minority Report, 1956Policymaking is about the future. If we were able to predictthe future accurately, preferred policies could be identified(at least in principle) by simply examining the future thatwould follow from the implementation of each possiblepolicy and picking the one that produced the most favorableoutcomes. However, for most systems of interest today(particularly social and economic systems), such predictionis not possible, due to their increasing complexity, theirincreasing interrelationships with other systems, and theincreasing uncertainty of developments external to thesystem that have important effects on the system. Wheneven the best model cannot reliably predict the details of asystem’s behavior, the classical approach of choosing apolicy based on the outcomes from a best estimate model isno longer credible. Such policies are ‘best’ for a future thatmost certainly will not occur, and have implications for thefuture that actually occurs that are typically not examined inthe course of policy design and analysis. Current approachesto policy analysis have serious difficulties in dealing withproblems characterized by complexity or disequilibriumbehavior, systems undergoing significant organizational andstructural change, and systems that can only be influencedrather than controlled. Yet, these characteristics haveincreasingly become staple characteristics of the world inwhich we live. Such systems are fundamentally unpredict-able. Yet, rapid economic, political, and social changes are areality, and public policies must be devised in spite ofprofound uncertainties about the future.Even though the future cannot be predicted, it is possibleto prepare for it. If, in the face of massive uncertainties,public policies are to be useful and credible, new approacheswill be needed for dealing with uncertainty. This specialissue of Integrated Assessment is a first step in filling thisneed. The papers in this issue are drawn from a sessionentitled ‘‘Dealing With Uncertainty in Policy Analysis andPolicymaking’’ that was part of the 5th InternationalConference on Technology, Policy and Innovation, whichwas held in The Hague in June 2001.There are five papers in this collection. They can bedivided into two categories:1. How can uncertainty in policy analysis and policymakingbe characterized (what is it? how can it be placed in ahistorical context? how can we classify different types ofuncertainties?)?2. How can policy analysts and policymakers deal withuncertainties (i.e., how can policies be developed thathave a good chance of succeeding in spite of enormousuncertainties about the future?)?1. CHARACTERIZING UNCERTAINTYThat uncertainties exist in practically all policymakingsituations is generally understood by policymakers andpolicy analysts. But there is little appreciation for the factthat there are many different types of uncertainty, and thereis a lack of understanding about their relative magnitudesand the different tools that are appropriate to use for dealingwith the different types. Even within the community ofpolicy analysts who deal with uncertainty in their work,there is no commonly shared terminology, and no agreementon a typology of uncertainties. The first paper (by Walker,Harrem€ooes, Rotmans, van der Sluijs, van Asselt, Janssen,and von Krauss) aims to provide a conceptual basis for thesystematic classification of uncertainty in model-baseddecision support activities, such as policy analysis, inte-grated assessment, and risk assessment. As van Asselt [1]notes, any typology of uncertainties is context dependent. Infact, according to her, uncertainty type by definition ‘‘refersto the way in which uncertainty manifests itself in aparticular context.’’ The context for the typology ofuncertainty presented in this paper is model-based decisionsupport. The authors first define uncertainty in model-based