In aerospace engineering, drag reduction is one of the most challenging problems of aircraft. Drag limits the maximum speed, the maximum range of flight and the operational cost. For example, reduction of the drag by a few percent can amount to billions of dollars of annual savings world wide on commercial aircraft fuel cost. There are three types of drags pressure or form drag, drag due to lift and skin-friction or viscous drag. The pressure drag involves the viscous influence upon the ideal or inviscidflow pressure field. It is generally small in the attached flow cases but increases tremendously when flow separation occurs. Therefore, the foremost consideration for drag control is to avoid flow separation, which in most cases can be achieved by simply streamlining the body shape. The drag due to lift is caused by flow spillage on lifting surfaces from highto low-pressure regions. Skin-friction drag is the result of the no-slip boundary condition on the wall and exists in both laminar and turbulent flows. It is equal to the integral of shear stress over the object surface area. Therefore the skin-friction drag in turbulent flow is higher than that in laminar flow. When the flow is attached, it is usually the major contributor to the total drag. For this reason, the reduction of skinfriction drag has been actively pursued by many researchers and will be the subject of study in this chapter.