The Lateral Torsional Buckling (L.T.B.) design of beams in buildings has received considerable attention over the last decades, and relatively similar improved design rules are now implemented in major design standards. On the contrary, it may be shown that the various available design specifications for the L.T.B. design check of bridge girders still lead to significant discrepancies regarding the reduction curve to be used. Furthermore, the resistance to L.T.B. of steel bridges depends on several specific parameters such as cross-bracings, geometric imperfections and residual stresses. The present paper investigates both experimentally and numerically the influence of these parameters on the resistance to L.T.B., and proposes improved design rules.