A DEM can be obtained from different topographic data sources and it represents the terrain topography, presenting deviations from reality, that is, errors, which sum to errors related to DEM processing. The nature and magnitude of these errors are not fully known and available to users for a given area of interest, making this lack of knowledge of the errors a source of uncertainty. This chapter analyzes the sources of uncertainty in DEM data and processing, discussing error propagation in river networks, watershed delineations, and hydrological outputs, and presents metrics and methods to evaluate the quality of DEM-derived products.