Composite sandwich structures find applications in many aerospace systems due to their lightweight and high strength. However, these structures are susceptible to low energy impact damage. Our studies of the damage resistance and tolerance of honeycomb core sandwich structures shows that the performance of the core plays a key role. Thus we have undertaken a study of the compressive and shear behavior of the aluminum honeycomb cores used in several space systems. The study consists of uniaxial and compression/shear tests of the core. Using a novel ring specimen loaded in compression and torsion, we can load along any line in compression/shear stress space in order to map out the core's yield surface and its evolution. Results from the experiments are used to validate computational models that are part of a larger simulation of the compression after impact strength of honeycomb core composites.