The erosion of solids caused by cavitating liquids is a result of the concerted collapse of clusters of cavities. In vibratory cavitation equipment the clusters grow and collapse adjacent to a solid surface and are typically of hemispherical or cylindrical form. In the present paper the collapse process of these clusters is described and the collapse equations are developed and solved. The theoretical results are compared with results from high-speed photography of the clusters and with the initial stages of cavitation erosion on metal specimens. Experimental and theoretical results show that the collapse of a cavity cluster is driven by the ambient pressure and the collapse proceeds from the outer boundary of the cluster towards its center. During the collapse the pressure at the inward-moving cluster boundary increases continuously, and at the cluster center it rises significantly above the ambient pressure. Therefore the collapse velocity of the individual cavities increases towards the cluster center, which explains that the erosion, being caused by the individual cavities, occurs predominantly in this region. Likewise, the pressure increase at the cluster boundary explains why materials of even high strength can be eroded by collapse of cavity clusters at atmospheric ambient pressure. Journal of Applied Physics is copyrighted by The American Institute of Physics.