Recent studies on turbulent friction drag reduction by wavy surfaces are reviewed. Special focus is laid upon numerical studies of fully developed flow in a channel having wavy surfaces. Both the surfaces driven by the flow (i.e., passive surfaces) and those driven by the external power (i.e., active surfaces) are considered. In addition, the drag reduction by traveling wave-like blowing and suction, which is closely related to the wavy surfaces, is also discussed in detail.