This chapter explores the different types of certainty because different evaluation practices are better suited to dealing with different kinds of uncertainty. Scenario thinking allows ex ante evaluation to support strategic thinking in the face of uncertainty, while at the same time bringing rigor and transparency into the judgments about the suitability, feasibility, and acceptability of a chosen approach. Impact assessment may be defined as the systematic analysis of the intended and unintended consequences of policy interventions, which are often conducted ex ante when proposing new policy interventions. The chapter highlights the thinking about science that underpins our arguments for a changed approach to ex ante evaluation, and this concerns the relationship between science and prediction. Evaluators have always faced challenges from their peculiar location between the world of scientific research and the world of decision making. European Union impact assessments are future-oriented assessments looking at two or more options.