The impeller speed at which air is first entrained from the surface of a stirred tank (NE) is an operational limit. Where air entrainment is desirable, it is a lower limit, but where air entrainment is detrimental it is an upper limit. This study (1) determines parameters which affect NE and (2) develops a mechanistic model of air entrainment. Experiments were conducted to determine the effect of impeller submergence, impeller diameter, baffle geometry, and the physical properties of the fluid on NE for an up-pumping (PBTU), and a down-pumping pitched blade turbine (PBTD). Mean and RMS velocity profiles were measured for selected cases using laser Doppler velocimetry (LDV). Using this data, air entrainment in stirred tanks and at other free surfaces is compared and is found to depend on the balance between gravity, surface tension and surface turbulence. There must be sufficient turbulence at the surface to overcome surface tension and form bubbles. The entrained bubble size is determined by the mean flow below the surface, which acts to pull the bubbles into the tank. It is shown that impeller variables, such as the power number, impeller speed and diameter, cannot predict the point of air entrainment at the surface. The key predicting variable is the ratio of u, the RMS velocity at the surface, to the mean downward velocity U. At the point of air entrainment, this velocity ratio just balances the physical properties of the fluid.