The elastic post-buckling behavior of rectangular plates in combined compression and bending is presented. The theoretical investigation begins with the Marguerre's compatibility equation and proceeds to obtain a solution by the energy method. The computer results are first compared with some available test results. Then the theoretical results are utilized to derive effective width formulas in combined compression and bending. Finally a family of post-buckling strength curves of box sections are presented that are calculated from the derived effective width formulas.