Unstable flow past cavities is grouped into fluid-dynamic, fluid-resonant, and fluid-elastic categories. Fluid-dynamic oscillations are attributable to instability of the cavity shear layer and are enhanced through a feedback mechanism. Fluid-resonant oscillations are governed by resonance conditions associated with compressibility or freesurface wave phenomena. Fluid-elastic oscillations are primarily controlled by the elastic displacements of a solid boundary. Experimental information and theoretical models for the frequencies and amplitudes of each of these categories of oscillations as well as a combination of them are discussed in detail. Effects of arrangements of cavities in groups and variations in cavity shape (rectangular, circular, triangular, whistle-type, Helmholtz-type) are reviewed. Means of attenuation of oscillations via geometrical modifications and mass addition are summarized. Finally, the effects of cavity oscillations on the time-mean cavity pressure, drag, and shear-layer characteristics are discussed.