The active and passive control of flow around a circular cylinder at subcritical Reynolds numbers was investigated experimentally. Periodic flow excitation emanating from a tangential slot milled in its surface delayed separation in two ways: (i) forcing a by-pass transition when applied at relatively small distances from the front stagnation point and (ii) modifying the separated shear layer delaying its separation from the cylinder. In the latter case the actuation was most effective when it was introduced near the 90 point from the forward stagnation location. The effect of tripping and actuation was investigated by measuring surface pressures and velocity profiles in the far wake. The effects of changing the inclination of the actuation relative the surface and to the oncoming stream (Le. the slot could face upstream or downstream 1. the excitation location with respect to regions most receptive to transition and to separation control on the cylinder. as well as the excitation amplitude and frequency were all quantified. In addition. a splitter plate was attached to the rear of the cylinder in order to reduce the size of the vortices being shed from it. The flow modification is a function of the plate length and the state of the flow, that determined the separation location. The addition of the splitter plate enabled the flow that separates from the cylinder to reattach. when excited effectively. Some effects of Reynolds number. roughness and periodic excitation are discussed.