Over the past two decades, methods and numerical techniques for the assessment of the wind energy resource over simple and complex terrain have developed considerably. The European regional study represents the most detailed of such efforts. As a part of this study, the Wind Atlas Analysis and Application Program (WAsP) was developed. WAsP is a PC based program and was developed for the purpose of wind climate estimation with special regard to wind energy applications. WAsP is now increasingly used worldwide in wind turbine site assessments. A brief description of WAsP is given and then it is applied to predict wind speed-ups over a smooth isolated hill. Its predictions compare well with the 'field data on the upwind side of the hill. But, at the lee side WAsP tends to overestimate wind speeds. The program is also employed to predict average mean-wind speeds for all directions for a number of sites in steep hilly terrain - the Port Hills of Christchurch. The results are compared to a unique set of field measurements and WAsP capabilities are investigated. WAsP predictions are reliable for sites with gentle slopes (less than 0.3 gradient) and smooth peaks. However, WAsP predictions for sites with more rugged slopes are 10% to 25% larger than measured field values. WAsP accuracy in predicting wind speeds (and hence wind energy distribution) is highly dependent on the accuracy of map data, the ruggedness at the site, and the level of atmospheric correlation between the reference and predicted sites. The sign and magnitude of the prediction error due to site ruggedness is proportional to the difference in ruggedness' between the predicted and the reference site. In order to quantify this error, a utility program (RIX) WAsP applied to the results 'from the Port Hills. The RIX program produces satisfactory results for the Port Hills, particularly, for more rugged sites where WAsP errors are maximum, e.g. Sugar Loaf. The RIX program can be used to determine the degree of ruggedness around a site. In this way, a wind engineer is able to use this quantified ruggedness, for a prevailing wind direction, to adjust wind energy predictions produced by WAsP.