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A digital terrain model (DTM) is needed to extract forest variables. Its accuracy affects that of these variables, and thus it has to be known. Airborne laser scanning (ALS) is being increasingly used to produce DTM. The potential of full-waveform ALS (FWALS), however, for DTM production in forest areas has not been thoroughly evaluated, particularly under adverse conditions of heterogeneous forest stands, such as the typically extensively managed eucalypt plantations of north-central Portugal studied here. In the study area of 900 ha, an exhaustive assessment of FWALS-derived DTM accuracy was carried out based on the field survey of 43 plots. In each of these circular plots of roughly 400 m2, the coordinates were measured for all trees and all prominent terrain points using Global Navigation Satellite Systems receivers and total station. In addition, the quality of the FWALS data was assessed by surveying 1 m × 1 m grids of points on three bare areas. The accuracy assessment measures are computed by assuming normal and nonnormal distributions of the differences between the h values of FWALS terrain points and those of control points at the same planimetric locations. While the accuracy of the FWALS data is as expected, around 3 cm, that of the DTM of the forested area exceeds expectations. In fact, the DTM accuracy of 12 cm is higher than that obtained in prior ALS studies carried out in less adverse conditions than those studied here. Thus, the current FWALS-based methodology to derive DTM appears highly suitable to extracting forest variables.