The dynamic stall characteristics of several airfoils have been investigated to assess the effects of compressibility, unsteadiness, and airfoil geometry on the onset of dynamic stall. For the purpose of understanding the physical phenomena in the range of low reduced frequency of practical importance, it is found that the flow before the onset of stall can be considered quasisteady and hence predicted using inviscid theory. For moderate Reynolds number, our analysis predicts the presence of a separation bubble at the airfoil leading edge and suggests that bubble bursting, or failure to reattach after the initial separation, is the onset mechanism. It is found in numerical simulation that at angles of attack close to static stall minor movements in transition point location can cause bursting of the separation bubble and that bursting is more susceptible to transition location in a locally supersonic flow than in subsonic flow. Finally, it is found that the delay between passage through the static stall angle and the onset of dynamic stall decreases with increasing unsteadiness. This result suggests that separation is being promoted rather than delayed and that the lift benefits from dynamic stall only for the duration of the separation process.