This study derives errors inherent in methods used to specify marine surface wind fields in hindcast studies of extreme environmental factors associated with extratropical storms. The northeastern Pacific Ocean is taken as the study basin, but results should apply to most Northern Hemisphere areas. Introduction As part of the selection process for environmental design data for offshore structures to be placed in regions exposed to severe storms, data on extreme waves, currents, and surge are generated, usually by the application of hydrodynamic models in historical hindcast studies. The need to describe accurately the temporal and spatial distribution of the surface over the sea arises both in calibrating the hydrodynamic models and in applying the models to long meteorological series to provide a data base of environmental extremes for statistical evaluation. Specification of the most accurate wind field description possible is especially crucial in the calibration studies. There, it is important to isolate errors in predictions of environmental factors (waves, currents, and surge) that are related to model physics or numerics from those errors introduced by deficiencies in the wind field. Of course, accurate windfield specification is desired in long-term historical simulations, though the sparsity pf early history meteorological data places an upper bound on attainable accuracy. In lieu of high accuracy, it becomes important to determine the errors that characterize historical wind field descriptions and the impact of those errors on the accuracy and reliability of environmental series generated by hindcast models. While much work has been reported recently on development, calibration, and application of hindcast models, the specification and validation of wind fields has received little attention, at least for extratropical storms. This study was designed specifically to determine the errors characteristic of alternative methods that have been used to specify marine surface wind fields in hindcast studies of extreme environmental factors associated with extratropical storms. The northeastern Pacific Ocean was taken as the test basin, but the results should apply to any extratropical region with a comparable meteorological data density (e.g., north Atlantic, North Sea, Bering Sea).