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The intention of this paper is to establish accurate values for the elastic critical moment of steel beams in several cases of loading and end-restraint conditions by using a Geometrically Exact Nonlinear Theory. The in°uence of warping and lateral rotation restraints is studied for four idealized support conditions. The results are compared with the ones derived from approximate theories, and in particular the Brazilian Code NBR8800:1986, the American Specification prAISC-LRFD:2003, and the European Prestandards prEN1993-1-1:2002 Stage 54 and prEN1999-1-1:2004 Stage 54. A parametrical analysis is performed for welded doubly-symmetric I-beams using the ¯nite element program PEFSYS for the usual range in conventional structures.