Layers of stone or rock are often applied around hydraulic structures to protect the sand bed from scouring. Determining the size of the rocks is complicated by the fact that the flow over a bed protection usually is non-uniform. This thesis describes two experimental series in which the stability of a single stone under non-uniform flows is studied in detail. From these experiments more understanding in the process of damage to bed protections is obtained. It is seen that besides the classical form drag, also small-scale accelerations in the flow attribute to the stone movement. In the last chapter a new stability model is developed for practical use.