The application of an externally applied pre-stress on impact properties is studied on polymethyl methacrylate (PMMA) organic glass. Samples are tested under equi-biaxial compression, simple shear and a combination of biaxial compression and shear. Equi-biaxial compression is shown to increase the threshold stress level for projectile penetration whereas shear pre-stress has a large effect on the overall energy absorbed during an impact. There is also an apparent interaction observed between compression and shear to dramatically increase the thresh-old stress. Pre-stressed laminates show an increase in damage area because of the unique formation of a secondary cone. Higher levels of compressive equi-biaxial prestress significantly increase the stress relaxation time because of the corresponding increase in hydrostatic stress. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 3704-3715, 2009