Studies on the stability of the amour layer (d’Angremond et. al. [1999] revealed the importance of density of placement. The current research focuses on the influence of the density of placement on the stability of cubes in a double armour layer and tetrapods and rocks in a single armour layer. The experiments were performed in the Laboratory of Fluid Mechanics of the Faculty of Civil Engineering and Geosciences at the Technical University Delft. A model of a breakwater was constructed in a wave flume. An increase in density of placement resulted in all cases in an increase of stability, except in the case of cubes. Cubes have the tendency to start behaving like a placed block revetment including the characteristic failure mechanisms like uplift and sliding. Tetrapods seem to be unsuitable for single layer armour layers due to the fact that the filter layer is easily attacked by the waves even when no tetrapod has been removed. Experiments on rock showed that vertically placed elements lead to a much more stable construction due to their self-repairing ability. Characteristic for rock is the piling up of elements under the waterline caused by the impact of collapsing waves. This lead to very low densities of placement higher on the slope. Existing damage criteria are not suitable for density of placement. In this research an effort has been done to create a damage criteria, which considers area of attack, density of placement and different failure mechanisms.