The control of dynamic stall by periodic excitation was studied on NACA 0012 and 0015 airfoils under incompressible conditions, by means of twodimensional leading-edge excitation slots. Timeresolved surface pressure measurements were phaseaveraged and integrated to yield aerodynamic coefficients, and total drag was reduced from a wake survey. The dynamic bubble bursting mechanism, evident on the statically stalling NACA 0012 airfoil, was evident under dynamic conditions and its burst rate was controlled by the pitch-rate. Stall could be controlled by "trapping" the bubble upstream of the excitation slot location. In contrast, the NACA 0015 trailing-edge stall was controlled by a qualitative improvement in the pressure recovery. NACA 0012 dynamic stall was significantly more severe, typically requiring higher excitation amplitudes for effective control of the moment excursions. Furthermore, different reduced frequency ranges were found to be effective for the different airfoils. NACA 0015 control effectiveness was not proportional to Cμ, possibly due to the existence of centrifugal instabilities in the leading-edge region.