Abstract: Using time resolved X-ray densitometry, time resolved 2-D void fraction flow fields for a variety of cavitating flows are examined. Example results from the cavitating flows formed at the apex of a wedge, the flow over a hydrofoil, and the flow in the wake of a cylinder are presented. In these cavities are obtained to identify the mechanisms of transition from closed partial cavities to open cavities exhibiting periodic shedding of large gas pockets. From the void fraction field measurements, two distinct types of cavity shedding mechanisms are identified: shedding associated with a re-entrant jet in the cavity closure region that produces intermittent shedding of smaller scale cavities, and large scale, periodic cloud shedding caused by the formation of a condensation shock within the high void-fraction flow in the separated region of partial cavitation. A discussion of the observed occurrence and properties of the shock wave, and its role in causing periodic shedding is presented based on the one-dimensional model of shock propagation in bubbly mixtures and the relationship between the transition to strong shedding as a function of the Mach number of the bubbly flow in the cavity.