A considerable number of flat slabs supported by columns need to be strengthened against punching shear. Reasons are increasing loads, construction or design errors, but also more stringent code requirements due to the increased knowledge gained in the past years. This paper shows the effects of bonding post-installed shear reinforcement into inclined holes drilled from the bottom of the slab. Laboratory tests have shown that this method not only increases the slab strength but also adds significant deformation capacity. A design method based on the critical shear crack theory is presented and compared to the test results. First practical implementations have proved that the proposed strengthening system is efficient, simple and economic.