The phenomenon is described of secondary buckling in plates that are clamped along the loaded edges and simply supported and stiffened along the adjacent unloaded edges. An experiment is described that is used to study secondary buckling of plates with these boundary conditions. Empirical data are presented to verify that the numerical analysis developed, which is based on the first-order shear deformable finite element method, accurately predicts the occurrence of secondary buckling. The analysis is then used to study the influence on secondary buckling of the displacement constraints on the unloaded edges. It has been found that the boundary conditions on the inplane displacements normal to the unloaded edges have a strong effect on the occurrence or the nonoccurrence of secondary buckling. The more constrained this displacement is, the lower the load is to cause secondary buckling. The influence of laminate orthotropy is also discussed. It is shown that the softer the plate is in the loading direction, the greater the tendency is to exhibit secondary buckling.