A new high order theory, referred to as the Extended High-Order Sandwich Panel Theory (EHSAPT), was formulated for orthotropic sandwich beams/wide panels with a general layout. This new theory accounts for the axial, transverse normal, and shear rigidity of the core.Validation of the present theory was performed for several structural analysis problems including: static loading, static instability (global buckling and wrinkling), free vibrations (natural frequencies), and dynamic loading (blast and impact).The accuracy of the theory was assessed by comparison with elasticity solutions and with experiment.It is shown that this new theory has superior accuracy over other available computational models, especially for sandwich beams/wide panel configurations with stiffer cores.