Today, a strategic dimension is associated to patents and to their traditional function of protecting innovations: that of blocking the route of innovation of competitors. We attempt to identify the determinants of the use of blocking patents, using the distinction between offensive and defensive strategies. This research, which combines deduction and induction, is based on the perception of French intellectual property managers and attorneys. The main determinants of the use of blocking patents appear to be firm size, the activity’s technological intensity, the type of legal system and the intellectual property culture.