The work described in this thesis deals with the elastic flexural-torsional buckling of rigid—jointed plane frames composed of I-section members. The starting point of the investigations is the study of the buckling of a simple portal frame. A method of analysis is developed, in which the differential equations for buckling and the boundary and compatibility equations are solved numerically. The theoretical predictions for two small portal frames are in close agreement with the experimental results. Since the end warping conditions in rigid angle I-section joints have a significant influence on the critical loads, these are investigated.