Composite micromechanics and macromechanics and the miniature Izod impact test are used to investigate the impact resistance of unidirectional composites. Several composite systems are examined both theoretically and experimentally. The composites are classified theoretically with respect to their impact resistance for longitudinal, transverse, and shear modes. Experimental results are reported only for Izod impact with the fibers either parallel or transverse to the cantilever longitudinal axis. Impact resistance design criteria which evolved during this investigation are used to design hybrid composites with improved impact resistance. This is illustrated theoretically and demonstrated experimentally. The results show that in situ fiber and matrix elongation to fracture, matrix modulus, fabrication process, fiber and void volume ratios, and microresidual stresses are variables which affect the impact resistance. The ranking of composite impact resistance on the basis of measured and predicted results is in excellent agreement.