This chapter highlights the role of different sources of uncertainty in migration forecasting and scenario-setting. We identify different dimensions of uncertainty and the limits of our knowledge with respect to migration concepts, definitions, data and measurement, and drivers, and also selected features, including the presence of regular migration patterns across space, time and the life course. We then discuss the knowable and unknowable aspects of migration, with a focus on complex configurations of migration drivers, human agency and policies. Based on our typology of migration uncertainty introduced in Chapter 2, we present practical ways of addressing the related challenges in setting migration scenarios and deriving migration forecasts. We conclude with recommendations for some possible pragmatic, good-enough solutions that can be implemented in practice.