Abstract
Extreme events like the credit crunch, the September 11th attacks, the coronavirus pandemic, and Hamas' attack on Israel each have in common that they should not have come as a surprise, yet still did. One reason surprises happen is that a risk assessment reflects the knowledge of the assessors, yet risk also includes uncertainties that extend beyond this knowledge. A risk assessment is thus susceptible to surprises as it focuses attention on what is known. Developing an expectation for surprises is key to their avoidance and requires that risk assessors specifically consider their "unknowledge"-that is, what they do not presently know about an event, outcome, or activity and its potential consequences and triggers. One way to emphasize the need for risk assessors to consider unknowledge is to explicitly include it as a separate component in risk-assessment frameworks. This article formalizes the inclusion of unknowledge in a contemporary risk-assessment framework.