In this paper we report on the development of a simple model for calculating the energy absorption by polymer composites upon ballistic impact. Three major components were identified as contributing to the energy lost by the projectile during ballistic impact, namely the energy absorbed in tensile failure of the composite, the energy converted into elastic deformation of the composite and the energy converted into the kinetic energy of the moving portion of the composite. These three contributions are combined in the model to determine a value for the ballistic limit of the composite, V₀. The required input parameters for the model were determined by a combination of physical characterization (for the physical and mechanical properties of the composites and the characteristics of the projectile) and from high-speed photography (for the size of the deformed region and the cone velocity). As the failure event usually occurred between two of a relatively small number of frames from the high-speed camera, the model predicted a range for V₀. This range of V₀ was compared with experimentally determined values for three composite systems: woven Nylon-66 fibres in a 50:50 mixture of phenol formaldehyde resin and polyvinyl butyral resin, woven aramid fibres in a similar matrix and Dyneema UD66 (straight gel-spun polyethylene fibres laid in a 0/90 fibre arrangement in a thermoplastic matrix). In all cases, the experimentally measured values of V₀ were found to lie within the range predicted by the model. The size of the deformed region, formed through shear deformation, on the back face of the composite was found to relate directly to the in-plane shear modulus of the material. Perhaps the most surprising result was that the dominant energy absorbing mechanism was found to be the kinetic energy of the moving portion of the composites.