© 2017 The effect of matrix shear strength on the ballistic response of simply-supported carbon fibre reinforced plastic (CFRP) beams was explored for a flat-ended projectile. To gain insight into the deformation and failure mechanisms, the following additional tests were performed on CFRP beams: (i) quasi-static indentation tests with rigid back support and, (ii) quasi-static cropping tests. In all 3 types of tests, CFRP [0°/90°] cross-ply laminates were tested in six states of cure, such that the matrix shear strength ranges from 0.1 MPa to 100 MPa. In the quasi-static cropping tests, the composite beams failed by shear plugging (involving transverse matrix cracks, ply delamination, and fibre fracture). In contrast, indirect tension (by ply tensile failure in the fibre direction) occurred in the back-supported quasi-static indentation tests. In the ballistic tests, the CFRP beams of high matrix shear strength (30 MPa–100 MPa) failed by a shear plugging mode. When the matrix shear strength was less than 30 MPa, the failure mode and the penetration velocity doubled and occurred by indirect tension. The optimal shear strength to give adequate static and ballistic strength is on the order of 20 MPa.