The inception of cavitation on isolated surface irregularities imbedded in a turbulent boundary layer is investigated experimentally and theoretically. Two families of cylindrical roughness elements having constant cross sections are studied. One family has a circular-arc cross section. The other family has a triangular cross section and was selected to simulate the separating flow which is typical of an actual surface irregularity. The theoretical minimum-pressure coefficient for the circular-arc irregularities is determined as a function of the relative height of roughness for several values of the boundary-layer shape parameter. Cavitation tests in the water tunnels of the Ordnance Research Laboratory on roughness elements ranging from 0.002 to 0.5 in. in height indicate that the incipient-cavitation number of an isolated surface irregularity is dependent upon the relative height of roughness, the boundary-layer shape parameter, the velocity, and other variables as yet unknown.