When a breakwater is under heavy wave attack, the concrete armour units will occasionally move, causing a collision between two concrete armour units. This process is called rocking, and induces stresses in the concrete, that may lead to breakage of the concrete armour units. This MSc Thesis provides a probabilistic method to predict breakage of concrete armour units, focussed on Xbloc®. The impact velocity is based on the forces on a unit under wave attack. This impact velocity is used as input to determine the impact force, based on an energy balance. The stresses at a critical location in armour unit will then be determined from a strut-and-tie model. With an estimation of the distribution of several stochastic variables, the eventual result is a probabilistic prediction of breakage of the concrete armour units.