Titel short: Ultimate Strength of a large WT-blade The present work contains a study of the structural static strength of wind turbine blades. A combination of experimental and numerical work has been used to address the most critical failure mechanims and to get an understanding of the complex structural behaviour of wind turbine blades. Four failure mechanisms observed during the full-scale tests and the corresponding FE-analysis are presented. Elastic mechanisms associated with failure, such as buckling, localized bending and the Brazier effect, are studied. In the thesis six different types of structural reinforcements helping to prevent undesired structural elestic mechanims are presented. Five full-scale tests have been performed. Comparisons of the ultimate loads observed in the full-sacle test are presented and conclusions are drawn based on the mechanisms found. The thesis consists of a comprehensive summery and a collection of three papers and four patent applications.