Impacts of climate change on hydrology have been widely studied in recent literature. Of large relevance in hydrology is the quantification of uncertainties involved in hydrologic projections. It is also acknowledged that hydrologic analyses and designs using assumptions of stationarity are no longer valid because of nonstationarities introduced by climate change. In this chapter, a broad overview of uncertainty modeling and methodologies for uncertainty quantification in assessment of hydrologic impacts of climate change is provided. Various sources of uncertainties arising in the impact assessment process are presented. Case studies of quantification, combination and propagation of uncertainty in impact assessment are discussed.