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In a broad sense, uncertainty can be simply defined as missing knowledge; ie, the absence of information. With respect to policymaking, uncertainty refers to the gap between available knowledge and the knowledge policymakers would need in order to make the best policy choice. This uncertainty clearly involves subjectivity, since it is related to satisfaction with existing knowledge, which is colored by the underlying values and perspectives of the policymaker (and the various actors involved in the policymaking process). Uncertainty can be associated with all aspects of a policy problem (eg, the system of interest, the world outside the system, the outcomes from the system, the weights stakeholders place on the various outcomes, etc.). There are myriad examples of how uncertainty plays havoc with public policies and policymaking. For example, in 1995, after a two-year deliberative process, some decisions were made by the Dutch Parliament that were intended to guide the growth of civil aviation in the Netherlands to the year 2015. One of the outcomes of the process was the decision to constrain the number of passengers at Schiphol to no more than 44 million passengers per year. This constraint was supposed to be more than enough to accommodate the most optimistic estimates of passenger growth until at least the year 2015. This limit was actually reached in 2004. And the noise limits, also expected to be reached no sooner than 2015, were reached in 1999. As a result, policymakers were forced to revisit their air transport policy (something they thought they would not have to do until 2015). Swanson et al.[1] describe a long-standing rate …